A 5.1 magnitude tremor was detected close to North Korea's nuclear test site on Wednesday, with Chinese earthquake monitors suggesting it could be the result of an atomic explosion.

The US Geological Survey said the epicentre of the quake -- detected at 10:00 am Pyongyang time (0130 GMT) -- was in the northeast of the country, some 50 kilometres (30 miles) northwest of Kilju city, placing it right next to the Punggye-ri nuclear test site.

The website of the China Earthquake Network Centre described it as a "suspected explosion".

The South Korean defence ministry said it was looking into the reports, while Yonhap news agency said Foreign Minister Yun Byng-Se had convened an emergency meeting.

"Further analysis is necessary to determine whether it is an artificial or a natural quake," a spokesman for the Korea Meteorological Administration told AFP.

The North has conducted three nuclear tests in 2006, 2009 and 2013 -- all at the Punggye-ri site.

Researchers at the US-Korea Institute at Johns Hopkins University said last month that recent satellite images showed North Korea was excavating a new tunnel at Punggye-ri.

"While there are no indications that a nuclear test in imminent, the new tunnel adds to North Korea's ability to conduct additional detonations over the coming years if it chooses to do so," they said at the time.