Seismic activity has been detected in North Korea, in what several nations say indicates that a widely-anticipated nuclear test has taken place.
A shallow 4.9 magnitude quake was detected near a known nuclear test site on Tuesday morning, the USGS said.
A UN monitoring agency described it as an “unusual seismic event” with “explosion-like characteristics”.
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 “consequences” if Pyongyang went ahead.
The USGS said the quake occurred at a depth of 1km (0.6 miles). China’s Earthquake Administration described it as a “suspected explosion”, AFP reported.
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.
Yonhap news agency says South Korea has raised its military alert. Japan says it will hold a meeting of its national security council, public broadcaster NHK reported.
North Korea is not prone to seismic activity. It conducted nuclear tests in 2006 and 2009, each time 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.