The US and Japan say they are looking into reports that North Korea has test-fired a short-range missile into the Sea of Japan.
N Korea has not launched long-range missiles since 1998
Japanese public broadcaster NHK said the missile had flown about 100km (62 miles) into the sea.
Tokyo has been monitoring the situation after earlier warnings that Pyongyang is close to testing a nuclear warhead.
North Korea has already developed long-range missiles that reach Japan and has pulled out of nuclear talks.
"It appears there was a test of a short-range missile by the North Koreans and it landed in the Sea of Japan," White House Chief of Staff Andrew Card told US media.
He said that he would not be surprised if the North Koreans had tested a missile.
"I think they are looking to be kind of bullies in the world and they're causing others to stand up and take notice," Mr Card said.
The missile launch would come a day before the 187 nations who have signed up to the international Non-Proliferation Treaty meet in New York to review its progress.
In March the North Korean government said it was no longer observing a self-imposed moratorium on long-range missile testing which had been in place since 1999.
Japanese news agency Kyodo said Tokyo had been informed by the US military of the North Korean test, believed to have been carried out at 0800 (2300 GMT on Saturday).
At a time when every move by North Korea has been scrutinised by its neighbours, the test-firing of even a short-range missile is a cause for anxiety, says the BBC's Jonathan Head in Tokyo.
No-one outside the secretive communist state has any clear idea what its intentions are, our correspondent says.
Map shows range of Taepodong 1 missile, flown over Japan in 1998. Range 1,500-2,000 km, payload: 1,000 kg
Evidence that North Korea working on testing Taepodong 2. Range up to 8,000 km (could reach western US)
Evidence from Jane's Defence of a pair of new ballistic missiles - one sea-based
Other missiles: Scud-B: Range 300 km, payload 1,000 kg
Scud-C: Range 500 km, payload 7600-800 kg
Scud-D (Nodong): Range 1,000-1,300 km, payload: 700-1,000 kg
US officials fear North Korea is now preparing to test its first nuclear device after its announcement in February that it already possessed nuclear weapons and that it would not be coming back to the six-party negotiations over its nuclear programme.
On Thursday, a senior US intelligence official told senators in Washington that North Korea now had the capability to arm its missiles with nuclear warheads, although he was not sure how quickly it could do so.
A full nuclear test would run the risk of alienating China and South Korea that are vital for North Korea's ruined economy, says our Tokyo correspondent.
But analysts believe it may still feel it has to go ahead to prove to itself and to the world that it has mastered the technology to join the nuclear club.
North Korea last launched a high-profile missile test in March 2003, to coincide with the inauguration of South Korean President Roh Moo-hyun.
These were two short-range land-to-ship missiles fired into the Sea of Japan.
It has not launched long-range missiles since 1998, when a Taepodong 1 missile flew over Japan.