A spokesman for the North’s General Department of Atomic Energy said that scientists will begin work at a uranium enrichment plant and a graphite-moderated 5 megawatt reactor, which generates spent fuel rods laced with plutonium and is the core of the Nyongbyon nuclear complex.

The unidentified spokesman said the measure is part of efforts to resolve the country’s acute electricity shortage but also for “bolstering up the nuclear armed force both in quality and quantity,” according to a statement carried by the official Korean Central News Agency.

Pyongyang conducted its third nuclear test in February, prompting a new round of U.N. sanctions that have infuriated its leaders and led to a torrent of threatening rhetoric. The United States has sent nuclear-capable bombers and stealth jets to participate in annual South Korean-U.S. military drills that the allies call routine but that Pyongyang claims are invasion preparations.

North Korea has declared that the armistice ending the Korean War in 1953 is void, threatened to launch nuclear and rocket strikes on the United States and, most recently, declared at a high-level government assembly that making nuclear arms and a stronger economy are the nation’s top priorities.

The threats are seen as efforts to force policy changes in Seoul and Washington and increase domestic loyalty to young North Korean leader Kim Jong Un by portraying him as a powerful military force.

“North Korea is keeping tension and crisis alive to raise stakes ahead of possible future talks with the United States,” said Hwang Jihwan, a North Korea expert at the University of Seoul. “North Korea is asking the world, `What are you going to do about this?”‘

North Korea added the 5-megawatt, graphite-moderated reactor to its nuclear complex at Nyongbyon in 1986 after seven years of construction. The country began building a 50-megawatt and a 200 megawatt reactor in 1984, but construction was suspended under a 1994 nuclear deal with Washington.

North Korea says the facility is aimed at generating electricity. It takes about 8,000 fuel rods to run the reactor. Reprocessing the spent fuel rods after a year of reactor operation could yield about 7 kilograms of plutonium — enough to make at least one nuclear bomb, experts say.

Nuclear bombs can be produced with highly enriched uranium or with plutonium. North Korea is believed to have exploded plutonium devices in its first two nuclear tests, in 2006 and 2009.

In 2010, the North unveiled a long-suspected uranium enrichment program, which would give it another potential route to make bomb fuel. Uranium worries outsiders because the technology needed to make highly enriched uranium bombs is much easier to hide than huge plutonium facilities.

But experts say plutonium is considered better for building small warheads, which North Korea needs if it is going to put them on missiles. Analysts say they don’t believe North Korea currently has mastered such miniaturization technology.

Scientist and nuclear expert Siegfried Hecker has estimated that Pyongyang has 24 to 42 kilograms of plutonium — enough for perhaps four to eight rudimentary bombs similar to the plutonium weapon used on Nagasaki in World War II.

It’s not known whether the North’s latest atomic test, in February, used highly enriched uranium or plutonium stockpiles. South Korea and other countries have so far failed to detect radioactive elements that may have leaked from the test and which could determine what kind of device was used.

“North Korea is dispelling any remaining uncertainties about its intention for developing nuclear arms. It is making it clear that its nuclear arms program is the essence of its national security and that it’s not negotiable,” said Sohn Yong-woo, a professor at the Graduate School of National Defense Strategy of Hannam University in South Korea.

“North Korea is more confident about itself than ever after the third nuclear test,” Sohn said. “That confidence is driving the leadership toward more aggressive nuclear development.”

A U.S. defense official confirmed to Fox News that the Navy had moved the U.S. destroyer to a location near South Korea. The ship, the USS McCain, is equipped with the Aegis defense system and is capable of shooting down missiles. Officials initially misidentified the ship but later clarified the USS McCain had been dispatched.

The defense official said the decision is not part of the ongoing joint military exercise with South Korea but part of the Navy’s general movements. The Navy had the same type of ship in this location as recently as December.

But, the positioning comes after the Pentagon dispatched two F-22 stealth fighter jets to South Korea to join Seoul forces in a training exercise, and after other demonstrations of U.S. capability last week.

White House Press Secretary Jay Carney said Monday that Kim Jong Un’s steadily escalating threats follow a “pattern,” but one that must be taken seriously. He said the U.S. is now taking “prudent” measures to “reassure our allies, demonstrate our resolve to the North and reduce pressure on Seoul to take unilateral action.”

This would include efforts to boost missile defense, last week’s rare flight of B-2 bombers to South Korea and likely the decision Sunday to deploy F-22 Raptors to Osan Air Base in South Korea.

With analysts warning that the biggest threat from North Korea is that it could start a war by going too far and provoking South Korea, Carney suggested the shows of strength are aimed at deterring the North from crossing that line.

“We believe this has reduced the chance of miscalculation and provocation,” Carney said of the B-2 flights and other actions.

Carney, though, said that despite the “harsh rhetoric” out of Pyongyang, the U.S. is not seeing changes to the country’s “military posture,” such as any “large-scale” mobilizations or positioning of forces.

“This pattern of bellicose rhetoric is not new, it is familiar,” Carney said. “We take it very seriously, we take prudent measures in response to it.”

The F-22 flight came after North Korea warned that the Korean Peninsula had entered “a state of war.”

A Pentagon spokesman confirmed to Fox News that the F-22 Raptors were deployed to Osan Air Base in South Korea from Japan on Sunday to support ongoing U.S.-South Korean military drills.

“This exercise has been planned for some time and is part of the air component of the Foal Eagle exercise,” spokesman George Little told reporters Monday.

Meanwhile, the North Korean leader gathered legislators Monday for an annual spring parliamentary session that followed a ruling party declaration that nuclear bomb building and a stronger economy were the nation’s top priorities.

The meeting of the Supreme People’s Assembly follows near-daily threats from Pyongyang, including vows of nuclear strikes on South Korea and the U.S.

North Korea said Saturday its armed forces, “will blow up U.S. bases for aggression in its mainland and in the Pacific operational theaters including Hawaii and Guam.”

Kim also threatened to shut down a border factory complex that is the last major symbol of inter-Korean cooperation.

The threats are seen as part of an effort to provoke the new government in Seoul to change its policies toward Pyongyang and to win diplomatic talks with Washington in order to gain more aid.

Fox News’ Justin Fishel and The Associated Press contributed to this report.

A senior U.S. official confirms to Fox News that the F-22 Raptors were deployed to Osan Air Base in South Korea from Japan on Sunday to support ongoing U.S.-South Korean military drills.

Meanwhile, North Korean leader Kim Jong Un gathered legislators Monday for an annual spring parliamentary session that followed a ruling party declaration that nuclear bomb building and a stronger economy were the nation’s top priorities.

The meeting of the Supreme People’s Assembly follows near-daily threats from Pyongyang, including vows of nuclear strikes on South Korea and the U.S. In a statement released Sunday, U.S. military in South Korea urged North Korea to restrain itself.

“(North Korea) will achieve nothing by threats or provocations, which will only further isolate North Korea and undermine international efforts to ensure peace and stability in Northeast Asia,” the statement said.

Meanwhile, North Korea said Saturday its armed forces, “will blow up U.S. bases for aggression in its mainland and in the Pacific operational theaters including Hawaii and Guam.”

Kim also threatened to shut down a border factory complex that is the last major symbol of inter-Korean cooperation, according to the Associated Press.

The threats are seen as part of an effort to provoke the new government in Seoul to change its policies toward Pyongyang and to win diplomatic talks with Washington in order to gain more aid.

The White House says the U.S. is taking North Korea’s threats seriously, but has also noted Pyongyang’s history of “bellicose rhetoric.”

On Thursday, U.S. military officials revealed that two B-2 stealth bombers dropped dummy munitions on an uninhabited South Korean island as part of annual defense drills that Pyongyang sees as rehearsals for invasion. Hours later, Kim ordered his generals to put rockets on standby and threatened to strike American targets if provoked.

Military analysts have said a full-scale conflict between North and South Korea is extremely unlikely, noting that the Korean Peninsula has remained in a technical state of war for 60 years. But the North’s continued threats toward South Korea and the United States have raised worries that a misjudgment between the sides could lead to a clash.

In addition to the military exercise, the U.S. will fortify its defenses against a potential North Korean missile attack by adding more than a dozen missile interceptors to the 26 already in place at Fort Greely, Alaska, Defense Secretary Chuck Hagel has also announced.

Fox News’ Justin Fishel and The Associated Press contributed to this report.

“From this time on, the North-South relations will be entering the state of war and all issues raised between the North and the South will be handled accordingly,” said a statement carried by the official North Korean news agency, according to a Reuters report.

The joint statement by the government, political parties and organizations said North Korea will deal with all matters involving South Korea according to “wartime regulations.” It also warned it will retaliate against any provocations by the United States and South Korea without “any prior notice.”

The divided Korean Peninsula is already in a technical state of war because the 1950-53 Korean War ended in a cease-fire, not a peace treaty. But Pyongyang said it was scrapping the war armistice earlier this month.

Reuters reported that North Korea’s statement said it would respond “without mercy” to any action by South Korea that harmed its sovereignty, indicating it was not about to mount a pre-emptive strike.

Analysts say a full-scale conflict is extremely unlikely and North Korea’s threats are instead aimed at drawing Washington into talks that could result in aid and boosting leader Kim Jong Un’s image at home. But the harsh rhetoric from North Korea and rising animosity from the rivals that have followed U.N. sanctions over Pyongyang’s Feb. 12 nuclear test have raised worries of a misjudgment leading to a clash.

South Korea’s Unification Ministry released a statement saying the latest threat wasn’t new and was just a follow-up to Kim’s earlier order to put troops on a high alert in response to annual U.S-South Korean military drills. Pyongyang sees those drills as rehearsals for an invasion; the allies call them routine and defensive.

In an indication North Korea is not immediately considering starting a war, officials in Seoul said South Korean workers continued Saturday to cross the border to their jobs at a joint factory park in North Korea that’s funded by South Koreans

On Friday, North Korean leader Kim Jong Un warned his forces were ready “to settle accounts with the U.S.” after two nuclear-capable U.S. B-2 bombers dropped dummy munitions on a South Korean island range as part of joint drills and returned to their base in Missouri.

North Korean state media later released a photo of Kim and his senior generals huddled in front of a map showing routes for envisioned strikes against cities on both American coasts. The map bore the title “U.S. Mainland Strike Plan.”

At the main square in Pyongyang, tens of thousands of North Koreans turned out for a 90-minute mass rally in support of Kim’s call to arms. Small North Korean warships, including patrol boats, conducted maritime drills off both coasts of North Korea near the border with South Korea earlier this week, South Korean Defense Ministry spokesman Kim Min-seok said in a briefing Friday. He didn’t provide details.

The spokesman said South Korea’s military was mindful of the possibility that North Korean drills could lead to an actual provocation. He said the South Korean and U.S. militaries are watching closely for any signs of missile launch preparations in North Korea. He didn’t elaborate.

Experts believe North Korea is years away from developing nuclear-tipped missiles that could strike the United States. Many say they’ve also seen no evidence that Pyongyang has long-range missiles that can hit the U.S. mainland.

Still, there are fears of a localized conflict, such as a naval skirmish in disputed Yellow Sea waters. Such naval clashes have happened three times since 1999. There’s also danger that such a clash could escalate. Seoul has vowed to hit back hard the next time it is attacked.

“The first strike of the revolutionary armed forces of the DPRK will blow up the U.S. bases for aggression in its mainland and in the Pacific operational theatres including Hawaii and Guam,” the North said Saturday in the statement carried by the official Korean Central News Agency. DPRK stands for the Democratic People’s Republic of Korea, the North’s official name.

Pyongyang uses the U.S. nuclear arsenal as a justification for its own push for nuclear weapons. It says that U.S. nuclear firepower is a threat to its existence.