South Korean scientists detected a high level of xenon gas -- a by-product of atomic tests -- two days after North Korea last month claimed it staged a successful nuclear experiment, officials said on Monday.
The North on May 12 announced it had conducted a nuclear fusion reaction, a claim greeted with scepticism in the South at the time because of the technical difficulty inherent in the fusion process.
Xenon is produced during the nuclear fission process.
Monday's Chosun Ilbo newspaper said the xenon readings taken near the border raised speculation the North had carried out some sort of small-scale nuclear test aimed at developing fusion technology.
"The amount of xenon detected in the air at the Geojin monitoring site in Goseong county on May 14 was eight times more than normal," a science and education ministry official told AFP on condition of anonymity.
The official said the ministry could not determine the reason for the abnormally high level of xenon, a gas that occurs in the atmosphere in trace amounts.
"There was no tremor -- which usually goes with nuclear explosions -- detected at the time," the official said. "Xenon may have come from a nuclear reactor elsewhere or due to a reason we have yet to fully understand."
The Korea Institute of Nuclear Safety, affiliated to the ministry, also confirmed the detection of a high level of xenon on May 14 but declined to elaborate.
South Korean scientists used xenon readings to confirm the North's first nuclear test on October 9, 2006. The North staged a second test in May 2009.
Nuclear fusion potentially promises clean and limitless energy but can also be employed to make hydrogen bombs.
The North's announcement last month did not link the alleged fusion breakthrough to its atomic weapons programme.
Pyongyang's official media reported on May 12 that the North's scientists, using their own technology, had conducted a nuclear fusion reaction.
"The successful nuclear fusion marks a great event that demonstrated the rapidly developing cutting-edge science and technology of (North Korea)," said party newspaper Rodong Sinmun.
South Korean experts doubted that the North -- which suffers persistent power shortages in everyday life -- had made major progress in the process.
Yang Hyung-Lyeol, of South Korea's state-funded National Fusion Research Institute, said at the time the North may have began operating a small-scale magnetic nuclear fusion device.
But Yang said there would be no comparison with a major project in which the South is involved -- the International Thermonuclear Experimental Reactor project to build a fusion power plant by the mid 2030s.