Washington says North Korea has declared a maritime exclusion zone off its coast in the Sea of Japan in a move which may indicate an imminent missile test.
American troops are on new exercises in South Korea
Pyongyang issued the warning for the 8-11 March to cover virtually the same area in which an anti-ship missile was tested on 25 February, a Pentagon spokesman said.
"We are certainly aware that they have filed a notice of exclusion - that is typically a precursor to a missile test," Navy Lieutenant Commander Jeff Davis told Reuters news agency.
Tension has been mounting in the region over the North's recent moves to restart its nuclear programme.
But the Pentagon spokesman said on Friday that the US was "not overly concerned".
North Korea tested its anti-ship missile on the eve of the inauguration of South Korean President Roh Moo-hyun.
Tension with the US, the South's main military ally, rose this week as Washington revealed that North Korean fighter jets had intercepted one of its surveillance planes.
The US recently boosted its military presence in the region by sending heavy bombers to its base on Guam.
Appeals for talks
US President George W Bush has called for a joint drive by his country and powers in the region to persuade North Korean President Kim Jong-il that "the development of a nuclear arsenal is not in his nation's interests".
North Korea is believed to have one or two nuclear weapons already, but there are fears it is preparing to restart a reprocessing plant which could allow it to produce five or six more.
Mr Bush said on Thursday that he wanted multilateral talks involving China, Russia, Japan and South Korea and added that the US had tried bilateral talks in the past but the North had not honoured its commitments.
However, Pyongyang followed his remarks with a new call for direct talks with Washington.
Washington has also indicated that it may redeploy some of its 37,000-strong garrison in South Korea, with Defence Secretary Donald Rumsfeld saying soldiers stationed near the border could be moved further south or withdrawn from the country altogether.
Mr Rumsfeld said the current positioning of US troops across the globe was largely a leftover from the Cold War, when troops were deployed to counter the perceived Soviet threat.
BBC Pentagon correspondent Nick Childs says Mr Rumsfeld now seems to be looking for a new more flexible approach to US military deployments worldwide.