North Korea is suspected of conducting a nuclear test, several nations say, after an earthquake that monitors said had "explosion-like" qualities.
North Korea says it has successfully carried out its third underground nuclear test.
A shallow 4.9 quake was detected at 11:57 (02:57 GMT), the USGS said.
Pyongyang said the test involved a miniaturised nuclear device and was carried out in a "safe and perfect manner".
Its epicentre was at in the same location as the Punggye-ri underground nuclear test site, at which activity has been observed in recent weeks.
The confirmation came three hours after seismic activity was detected at North Korea's nuclear test site.
There has been no word yet from North Korea, but it announced plans for a third nuclear test last month.
The UN Security Council had warned of "significant consequences" if Pyongyang went ahead.
The UN Security Council had warned of "consequences" if Pyongyang went ahead.
It is to hold an emergency meeting at 14:00 GMT on Tuesday in New York, diplomats say.
'High-level'
The USGS said the quake occurred at a depth of 1km (0.6 miles). A UN monitoring agency described it as an "unusual seismic event" with "explosion-like characteristics".
"Its location is roughly congruent with the 2006 and 2009 DPRK [North Korea] nuclear tests,'' said Tibor Toth, executive secretary of the Comprehensive Nuclear Test Ban Treaty Organisation.
Japan's Meteorological Agency said the tremor was "different from a normal earthquake".
"We believe that there is a possibility that North Korea carried out a nuclear test, looking at past cases," said Chief Cabinet Secretary Yoshihide Suga - an opinion echoed by South Korea's defence ministry.
South Korea has raised its military alert and outgoing President Lee Myung-bak has convened a meeting of the National Security Council. Seoul is also reported to have requested an emergency meeting of the UN Security Council.
Japan's government is also holding an emergency meeting, public broadcaster NHK reported.
North Korea is not prone to seismic activity. Its two previous nuclear tests were both conducted shortly after rocket launches condemned by the UN as disguised tests of long-range missile technology.
It announced in January that it planned to conduct a "high-level" nuclear test, in response to expanded UN sanctions imposed after its latest rocket launch on 12 December 2012.
That rocket launch successfully put a satellite into orbit, in an apparent breakthrough for the North.
The US, South Korea and Japan had all warned Pyongyang not to go ahead with the test. China, North Korea's closest ally and biggest trading partner, had also called for restraint.
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