North Korea likely took meticulous steps to conceal any residue from its February nuclear weapon test, fueling suspicions that it is using a new bomb design with highly enriched uranium at its core, The Washington Post reported Sunday.

Citing unnamed US officials and weapons experts, the newspaper said the effects of the February 12 explosion were remarkably well contained, with few radioactive traces escaping into the atmosphere.

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 paper said.

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

According to the paper, 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 cooperation between North Korea and Iran, the paper said. Iran has been concentrating on uranium enrichment.

The United States is concerned about last year's agreement between North Korea and Iran pledging technical and scientific cooperation, The Post said.

The pact was signed in Tehran in September at a ceremony attended by Iranian President Mahmoud Ahmadinejad and Fereydoun Abbasi, the head of Iran's nuclear program.