Nuclear proliferation experts are urgently researching an apparent earthquake in North Korea that struck 31 miles from a previous nuclear test site.
Chinese seismologists said the magnitude 3.4 quake was caused by a man-made blast, but South Korean officials said it could be a natural quake and not another nuclear test.
Lassina Zerbo, of the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO), said in a Twitter post that analysts were working to establish which it was.
"Korean Peninsula unusual #seismic activity: LAT=41.36 LON=129.76 mb=3.5 About 50km from prior tests. #CTBT Analysts investigating," he said.
The US Geological Survey (USGS), meanwhile, said the cause could not yet be determined.
China's earthquake administration announced on Saturday morning that it had detected the quake in North Hamgyong province, North Korea.
It gave the cause as "suspected explosion" and said it struck at a depth of zero kilometres, raising fears that Pyongyang might have conducted another nuclear bomb test.
Dictator Kim Jong-Un's last known nuclear test on September 3 caused an earthquake at the much higher magnitude of 6.3, drawing widespread condemnation at the UN.
North Korea has described that incident as a test of an advanced hydrogen bomb for a long-range missile, marking a dramatic escalation of the regime's stand-off with the United States and its allies.
The CTBTO was founded in 1996 to monitor compliance with an accord negotiated in the 1990s that bans nuclear explosions.
The treaty has not come into effect because eight countries with nuclear technology have yet to sign it.