South Korea has detected an "artificial earthquake" at North Korea's nuclear test site, Yonhap news agency reported Tuesday, suggesting Pyongyang had gone ahead with a threatened atomic test.
Yonhap said an "artificial earthquake of 5.1 magnitude" had been detected in Kilju county, where the Punggye-ri test site is located.
The US Geological Survey measured it as a 4.9-magnitude quake at a very shallow depth of just one kilometre (0.6 miles).
The South Korean defence ministry and the presidential Blue House both said they were trying to verify whether a nuclear detonation had taken place.
Pyongyang has been threatening a "higher-level" nuclear test for weeks despite warnings of severe repercussions from the UN Security Council.
If confirmed, it would mark the third time the North has detonated a nuclear device, following previous tests in 2006 and 2009.