SEOUL, South Korea — Kim Jong Un called nuclear weapons development one of North Korea’s top priorities as his country ratcheted up tensions by declaring a state of war with South Korea and reiterating threats to attack the United States.

Nuclear arms can “never be abandoned” nor “traded with billions of dollars,” Kim said Sunday at a meeting of the ruling Workers’ Party Central Committee, the state-run Korean Central News Agency reported. North Korea’s rubber stamp parliament meets Monday to ratify his remarks.

Tensions have escalated since North Korea detonated a nuclear device in February, denounced tightened United Nations sanctions and threatened pre-emptive nuclear strikes in response to U.S.-South Korea military drills.

The United States Sunday said it takes North Korean threats seriously while denouncing the “long history of bellicose rhetoric.”

The Obama administration Sunday sent F-22 Raptor fighter jets to South Korea to reinforce its commitment to defending its ally. While North Korea said March 30 it may shut the jointly run Gaeseong industrial zone in response to recent flights over the Korean peninsula by U.S. stealth bombers, South Korean workers crossed the border into the area Monday.

About 200,000 North Koreans — including workers and their families — depend on the Gaeseong industrial zone for income, according to Yang Moo Jin, a professor at the University of North Korean Studies in Seoul. The North generates about $100 million profit annually from the joint project and the South makes quadruple that amount, he said.

‘Last Resort’

“It seems there are no more cards left for North to pressure South now, and Gaeseong seems to be the last resort,” Yang said.

More than 350 South Koreans crossed the border Monday at 8:30 a.m. for work in Gaeseong, the Unification Ministry said in a text message. A total of 853 are expected to enter and 355 others to return from the complex Monday, ministry spokesman Kim Hyung Suk told reporters in Seoul.

The political bureau of North Korea’s sole political party Sunday unanimously endorsed building a light water reactor to help ease electricity shortages, and called for promoting international investment and foreign trade, KCNA said.

North Korea’s economy is about one-fortieth the size of South Korea’s and the country relies on China for diplomatic and economic support. Chronic food insecurity and malnutrition affect about two-thirds of the country’s 24 million people, according to a U.N. assessment last June.

War Footing

South Korean President Park Geun Hye has promised economic support if Kim abandons the nuclear weapons program, and last week approved 680 million won ($611,000) of tuberculosis medicine to be sent to the North, in the first shipment of humanitarian aid since she took office in February.

The totalitarian regime said March 30 it is in a “state of war” with South Korea, days after cutting off a cross-border hotline and putting its rocket forces on standby to strike the American mainland.

Analysts including Greg Thielmann, a senior fellow at the Arms Control Association in Washington, have said North Korea is years away from deploying missiles that could reach the continental United States.

Kim met with military leaders and ordered the preparations after two U.S. B-2 stealth bombers flew over South Korea on March 28 in a show of deterrence. Earlier flights by B-52 bombers were part of a two-month annual exercise that runs until the end of April.

‘Serious’ Threats

“We take these threats seriously and remain in close contact with our South Korean allies,” White House spokeswoman Caitlin Hayden said in an emailed statement on Mar. 30. “But we would also note that North Korea has a long history of bellicose rhetoric and threats” and the latest statement “follows that familiar pattern.”

Park Monday reaffirmed her intention to respond “firmly” to any North Korean attacks.

“If any provocations occur against our people and the nation, we must strongly retaliate in the beginning, without a single thought for political considerations,” she told military officials Monday in Seoul.

Japan, which hosts about 40,000 American troops, condemned a North Korean warning that it may attack U.S. bases on Okinawa and the Japanese mainland. The Rodong Sinmun, the Workers’ Party newspaper, Sunday said the bases “are within the striking distance,” KCNA reported.

‘Provocative Action’

“We strongly urge them to refrain from any provocative action,” Japanese Chief Cabinet Secretary Yoshihide Suga told reporters Monday in Tokyo. “Japan is doing everything possible to gather information and continue taking all precautions to ensure safety.”

The last time tensions were at this level between the two sides was in 2010, following the sinking of a South Korean warship that killed 46 sailors, and North Korea’s shelling eight months later of a South Korean border island, in which four people died.

The two Koreas remain technically at war because their 1950-1953 conflict ended without a peace treaty.

“It’s part of what I call March Madness on the Korean Peninsula,” said Kenneth Quinones, professor of Korean studies at Akita International University in Japan. “Every March when the U.S. and South Korea hold military maneuvers, North Korea goes on full alert and makes similar threats. The only difference is the rhetoric has intensified, and the situation certainly merits concern and close monitoring.”

No unusual North Korean troop movements have been detected since the threat to close Gaeseong and emergency communication channels with South Koreans in the complex are operating, Unification Ministry spokesman Kim Hyung Suk said on March 30.

Korean Cooperation

Gaeseong is the last remaining example of inter-Korean cooperation. More than 120 South Korean companies employ about 53,000 North Koreans at the complex. From its opening in 2005 through Jan. 31, goods totaling more than $2 billion have been produced there, according to Unification Ministry data.

About 900 South Koreans regularly visit Gaeseong, located about 10 miles (16 kilometers) north of the demilitarized zone, the world’s most heavily fortified border.

North Korea’s rhetoric has had little impact so far on South Korean stocks. The benchmark Kospi index was down 0.4 percent as or 12:51 p.m. in Seoul after gaining 2.9 percent last week, its best performance in six months.