SEOUL, South Korea — South Korean and U.S. troops facing North Korea boosted their alert level Thursday to the highest category since 2006, after the communist regime threatened military strikes on allied troops in escalating tensions over its nuclear test.
North Korea threatened Wednesday to attack any U.S. and South Korean ships that try to intercept its vessels and renounced a 1953 truce halting the Korean War fighting, raising the prospect of a naval clash off the Korean peninsula's west coast.
The North was responding to Seoul's decision to join a U.S.-led anti-proliferation program aimed at stopping and inspecting ships suspected of transporting banned weapons, including nuclear technology. South Korea announced it was joining after the North's underground test blast of a nuclear bomb.
On Thursday, the South Korea-U.S. combined forces command increased the surveillance to level 2 from the present level 3, Defense Ministry spokesman Won Tae-jae said. He said that was the highest level since 2006, when the North conducted its first-ever nuclear test.
The U.S. has 28,500 troops in South Korea as a deterrent against North Korea.
Won said the bolstered level means more aviation surveillance assets, intelligence analysts and other intelligence-collecting measures would be deployed to watch North Korea. He refused to disclose further details.
The North has long warned it would consider the South's participation in the U.S.-led Proliferation Security Initiative as a declaration of war against North Korea.
The North would "deal a decisive and merciless retaliatory blow" to anyone trying to inspect its vessels, according to a North Korean military statement, carried by the official Korean Central News Agency on Wednesday.
Key world powers, meanwhile, have proposed a range of expanded U.N. sanctions against North Korea in response to its nuclear as well as measures to give teeth to existing bans and ship searches against the reclusive country, a U.N. diplomat said Wednesday.
The five permanent veto-wielding council members _ the United States, Russia, China, Britain and France _ and the two countries most closely affected by the nuclear test, Japan and South Korea, discussed possible U.N. sanctions and other measures for a new Security Council resolution on Tuesday.
The diplomat, who is familiar with the talks but spoke on condition of anonymity because they were closed, said there was a clear commitment to go for sanctions in the new resolution and no reluctance from North Korea's allies, China and Russia. But what measures the 15-member council ultimately agrees to remains to be seen.
U.S. Secretary of State Hillary Rodham Clinton also said that North Korea faces consequences for its nuclear and missile tests and denouncing its "provocative and belligerent" threats. She also underscored the firmness of the U.S. treaty commitment to defend South Korea and Japan, which are in easy range of North Korean missiles.
Pyongyang lashed out at both the U.S. and South Korea, calling Seoul's move to join the Proliferation Security Initiative tantamount to a declaration of war and a violation of the truce keeping the peace between the two Koreas.
"Full participation in the PSI by a side on the Korean Peninsula where the state of military confrontation is growing acute and there is constant danger of military conflict itself means igniting a war," North Korea's Committee for the Peaceful Reunification of Korea said in a statement carried on state media.
North Korea's army said it would be "illogical" to honor the 1953 armistice between the two Koreas, given the violations by the U.S. and South Korea, and said it could no longer promise the safety of U.S. and South Korean warships and civilian vessels in the waters near the maritime border.
Clinton said North Korea has made a choice to violate U.N. Security Council resolutions, ignore international warnings and abrogate commitments made during six-nation nuclear disarmament talks.
"There are consequences to such actions," she said, referring to discussions in the United Nations about punishing North Korea for its nuclear and missile tests.
She did not provide specifics, saying only that the intent of diplomats was to "try to rein in the North Koreans" and get them to fulfill commitments made in the nuclear talks.
Clinton said she was pleased by a unified international condemnation of North Korea that included Russia and China, North Korea's closest major ally and the host of the currently stalled disarmament talks.
Despite her tough words, Clinton held out hope that North Korea would return to nuclear disarmament talks and that "we can begin once again to see results from working with the North Koreans toward denuclearization that will benefit, we believe, the people of North Korea, the region and the world."
At the White House, spokesman Robert Gibbs played down North Korea's angry rhetoric, saying the threats will only add to its isolation.
He said North Korea has threatened to end the armistice many times in the past but the peace has held.
The Russian Foreign Ministry said it voiced "serious concern" about the nuclear test to the North Korean ambassador and urged Pyongyang to respect the U.N. resolutions and return to the disarmament talks.
The truce signed in 1953 and subsequent military agreements call for both sides to refrain from warfare, but don't cover waters off the west coast. North Korea has used the maritime border dispute to provoke two deadly naval skirmishes _ in 1999 and 2002.
North Korea now is believed to have enough plutonium for at least a half-dozen weapons, but experts say it still has not mastered the miniaturization technology required to mount a nuclear warhead on a long-range missile.
After firing a long-range missile on July 4, 2006, and carrying out its first nuclear test three months later, North Korea agreed in February 2007 to start disabling Yongbyon in exchange for 1 million tons of fuel oil and other concessions. Disablement began in November 2007.
The process halted last summer in a dispute with Washington over verifying past atomic activities, and Pyongyang said last month it was quitting the talks altogether.
Associated Press writers Hyung-jin Kim in Seoul, and Foster Klug, Pamela Hess in Washington and Edith M. Lederer at the United Nations contributed to this report.