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

U.S. Defense Secretary Robert Gates said that the situation is worrisome but has 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.

About 28,000 U.S. troops are stationed in South Korea.

"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.

The Army's top officer, Gen. George Casey, expressed confidence that the U.S. 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 strong 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 is 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 will take time because the issues are "complicated" and that there are many suggestions.

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

In response to the nuclear test, South Korea said that 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." *