The United States said it is sending a missile defense system to Guam to defend it from North Korea.
US Defense Secretary Chuck Hagel described the situation as a "real and clear danger" from Pyongyang.
Earlier, Pyongyang said it had "formally" informed the Pentagon that it had "ratified" a possible nuclear strike.
It was unclear how such a warning was given since North Korea does not have diplomatic ties with Washington.
Its state news agency carried a statement from an army spokesman, saying a "merciless" attack against the US had been approved, potentially involving a "diversified nuclear strike."
North Korea has threatened a nuclear strike on the US and missile attacks on its Pacific bases, including in Guam, a US territory in the Pacific.
New UN sanctions were imposed over the country's third nuclear weapons test in February.
Yesterday, North Korea said it would revive a mothballed nuclear reactor able to produce bomb-grade plutonium.
"Some of the actions they've taken over the last few weeks, present a real and clear danger," Mr Hagel told an audience at the National Defense University in Washington.
Mr Hagel said he had to take threats seriously, language he has used in recent weeks as the United States revamped its missile defence plans and positioned two guided-missile destroyers in the western Pacific to bolster missile defence.
The United States has also flexed its muscle during military drills with South Korea last week, flying two bat-winged stealth bombers on a first-of-its-kind practice bombing run over South Korea.
North Korea this morning closed access to a joint factory zone that earns €1.6bn a year in trade for the impoverished state.
However, more than 30 South Koreans returned home in the evening, relieving fears they could have been held hostage.
Factories in the Kaesong Industrial Park are still believed to be operating, but North Korea's decision to block entry is a further sign of the growing tension on the Korean peninsula.
Yesterday, North Korea said it would restart a nuclear reactor that had been shut down in 2007.
The industrial park has not formally stopped operations since it was inaugurated in August 2000 as part of efforts to improve ties between the two Koreas.
South Korean companies pay a total of more than €62m a year in wages to workers in the zone.
It houses 123 companies and is staffed by 50,000 North Koreans and hundreds of South Korean business owners and managers.
More than 800 South Koreans had stayed overnight in the park, just north of the world's most heavily armed border.
South Korea's Unification Ministry demanded the park be opened.
North Korea has threatened a nuclear strike on the United States and missile attacks on its Pacific bases after fresh UN sanctions were imposed for the country's third nuclear weapons test in February.
The government has also said it is in a state of war with South Korea.