Nuclear proliferation watchdog CTBTO is examining unusual seismic activity in North Korea that took place around 50 km (31 miles) from previous nuclear testing in the isolated country, it said on Saturday.
"Analysts looking at unusual #seismic activity of a much smaller magnitude in the #DPRK," CTBTO Executive Secretary Lassina Zerbo said in a Twitter post.
"Korean Peninsula unusual #seismic activity: LAT=41.36 LON=129.76 mb=3.5 About 50km from prior tests. #CTBT Analysts investigating," he said in a subsequent post.
China's earthquake administration said it had detected a magnitude 3.4 quake in North Korea that was a "suspected explosion", raising fears that Pyongyang might have conducted another nuclear bomb test.
A CTBTO spokeswoman said Zerbo's remark about a "smaller magnitude" referred to its monitoring of an event in North Korea on Sept. 3 which the agency described as consistent with a man-made explosion but stopped short of calling a nuclear blast, pending testing for airborne radioactivity.
North Korea has described the Sept. 3 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 (Preparatory Commission for the Comprehensive Nuclear Test Ban Treaty Organization) 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.
