South Korean, U.S. and Japanese seismic monitoring agencies put the magnitude of Tuesday's quake between 4.9 and 5.2.
"We think it is possible it came about as a result of a nuclear test by North Korea from looking at past cases," Japanese government spokesman Yoshihide Suga said at a news conference Tuesday, noting that they were still gathering information.
"A natural earthquake normally starts with a smaller tremor followed by a larger one. This quake's strength was the same throughout," according to Yosuke Igarashi, an official at the Japan Meteorological Agency. He declined to elaborate on the length of the quake or other details, saying the agency was studying the data.
The United States and its allies have been on edge since North Korea said last month it will conduct its third nuclear test to protest toughened sanctions over the December rocket launch.
In Washington, the White House had no immediate comment on Tuesday's developments.
North Korea's politburo vowed to continue firing "powerful long-range rockets," but a statement by state media Tuesday made no mention of a nuclear test.
North Korea's National Defense Commission said Jan. 23 that the United States was its prime target for a nuclear test and long-range rocket launches. North Korea accuses Washington of leading the push to punish Pyongyang for its December rocket launch.
Last October, a spokesman from the commission told state media that the country had built a missile capable of striking the United States, but did not provide further details. A missile featured in an April 2012 military parade appeared to be an intercontinental ballistic missile, but its authenticity has not been verified by foreign experts.

DONATE HERE

update:
North Korea has threatened to fire a missile with a nuclear warhead into US territory.