WASHINGTON - North Korea appears to have taken unusual steps to conceal any residue from its February nuclear weapon test, fuelling suspicions that its scientists shifted to a bomb design that uses highly enriched uranium as the core, according to US officials and weapons experts.

They said the effects of the February 12 explosion were remarkably well contained, with few radioactive traces escaping into the atmosphere, according to a report in The Washington Post.

The US government anticipated the nuclear test, North Korea’s third, and monitored it closely for clues about the composition of the bomb, the report said.

But in the days following the detonation, US and South Korean sensors failed to detect even a trace of the usual radioactive gases in any of the 120 monitoring stations along the border and downwind from the test site, the Post said.

A Japanese aircraft recorded a brief spike of one radioactive isotope, xenon-133, but it was seen as inconclusive, it said.

According to the newspaper, the absence of physical data could suggest a deliberate attempt by North Korea to prevent the release of telltale gases, presumably by burying the test chamber deep underground.

In its first two nuclear tests, North Korea was thought to have used plutonium extracted from a stockpile of fissile material that the country developed in the late 1990s, the report said.

A successful test of a uranium-based bomb would confirm that Pyongyang has achieved a second pathway to nuclear weapons, using its plentiful supply of natural uranium and new enrichment technology, The Post said.

A device using highly enriched uranium would also deepen worries about co-operation between North Korea and Iran, the paper said. Iran has been concentrating on uranium enrichment.