The U.S. Defense Department says North Korea has declared a maritime exclusion zone in a portion of the Sea of Japan, a move officials say could signal a new missile test.
Pentagon officials say North Korea has warned ships to stay out of an area off its coast in the Sea of Japan.
Navy Lieutenant Commander Jeff Davis says the warning could be "a precursor to some sort of missile test."
Commander Davis says the Pentagon is "not overly concerned" about the announcement, but called it "not helpful," given the current level of tensions in the Pacific.
North Korea's notice to mariners is nearly identical to one announced before it tested an anti-ship missile last month.
That test came on the eve of the inauguration of South Korean President Roh Moo-hyun, and amid concerns over Pyongyang's refusal to abandon its nuclear weapons program.
U.S. officials downplayed that test, saying it involved a small missile and not one of North Korea's stock of longer-range ballistic missiles.
Tensions escalated recently when North Korean warplanes intercepted a U.S. surveillance plane over the Sea of Japan.
The fighters approached to within 15-meters, and while at least one jet locked on to the American aircraft, it did not open fire.
A White House official called the move provocative and reckless.
Before that incident, U.S. Defense Secretary Donald Rumsfeld ordered 12 B-52 bombers and a dozen B-1 bombers transferred to a U.S. base in Guam, which is closer to North Korea than the plane's bases in the United States.
Pentagon officials say the latest U.S. moves are a warning to North Korea not to underestimate U-S military capabilities, despite the current focus on Iraq and the massive buildup of American forces in the Persian Gulf.
North Korea wants the Bush administration to defuse the crisis over Pyongyang's nuclear program by holding direct talks.
President Bush says he wants to resolve the standoff through multi-lateral diplomacy.