SEOUL, South Korea - The United States and South Korea put their military forces on high alert yesterday after North Korea renounced the truce keeping the peace between the two Koreas since 1953 and threatened military action following nuclear and missile tests.

Defense Secretary Robert M. Gates said the situation was worrisome but had not reached a crisis level that would warrant additional U.S. troops in the region. Any military actions would need to be decided upon by broad international agreement, he said.

"I don't think that anybody in the [Obama] administration thinks there is a crisis," Gates told reporters aboard his military jet early this morning, still last night in Washington.

The Army's top officer, Gen. George Casey, expressed confidence that the United States could fight a conventional war against North Korea if necessary, despite continuing conflicts elsewhere.

North Korea conducted an underground nuclear explosion and a series of short-range missile test launches this week, drawing international condemnation. The U.N. Security Council has been discussing how to punish the regime for Monday's blast that President Obama called a "blatant violation" of international law.

Russia's U.N. ambassador said yesterday that there was wide agreement among key world powers on what a new U.N. resolution should include to respond to North Korea's second nuclear test, which violated a Security Council ban.

But Vitaly Churkin cautioned that putting the elements together and getting agreement would take time because the issues were "complicated" and there were many suggestions.

A list of proposals was compiled and sent to the seven governments 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.

Diplomats said a draft of the proposed resolution was not expected to be circulated until next week.

In response to the nuclear test, South Korea said it would join more than 90 nations that have agreed to stop and inspect vessels suspected of transporting weapons of mass destruction.

North Korea called South Korea's participation in the U.S.-led Proliferation Security Initiative a prelude to a naval blockade and a violation of the truce signed to end the three-year war that broke out in Korea in 1950. It also renounced the 1953 armistice and threatened to strike any ships trying to intercept its vessels.

"The northward invasion scheme by the U.S. and the South Korean puppet regime has exceeded the alarming level," the North's main Rodong Sinmun newspaper said in a commentary carried by the official Korean Central News Agency. "A minor accidental skirmish can lead to a nuclear war."

South Korean and U.S. troops yesterday raised their alert to the highest level since 2006 when North Korea carried out its first nuclear test. About 28,000 American troops are stationed across the South.

The two Koreas technically remain at war because they signed a truce, not a peace treaty, in 1953.

The South moved a 3,500-ton destroyer into waters near the western maritime border while smaller, high-speed vessels were keeping guard at the front line, the Yonhap news agency said.

North Korea, meanwhile, positioned artillery along the west coast on its side of the border, Yonhap reported.