The group consisted of officials from the missions of Britain, Germany, Poland, Russia, Sweden, the Czech Republic, India and Mongolia, with British ambassador David Slinn leading the team.

"He left about 8 o'clock this morning (2300 GMT Wednesday)," a British diplomat in the North Korean capital told AFP.

He added the group was scheduled to be flying for one hour in a small propeller plane and then travel by road for about three hours to the site.

But with torrential rain pounding many areas of North Korea in recent days, ruining farmland and destroying roads, it is not certain that the delegation will make it.

"There is such a fear," said Polish Ambassador to North Korea Wojciech Kaluza, who sent his deputy. "The weather at the location is not good today."

If everything went as scheduled, they should be back in Pyongyang by 6:00 pm (1000 GMT) but "there's a possibility they might not return today at all", he added.

"There's no set agenda," Kaluza told AFP. "Most of the time will be travelling. The point is to assess what happened."

The blast reportedly occurred last week in rugged Kimhyungjik county in Ryanggang province close to the Chinese border, kicking up a massive mushroom cloud in an area known to have storage for missiles and explosives.

South Korea's state-financed Korea Aerospace Research Institute released satellite pictures of the scene but they were too unclear to give any clues about the cause of the blast.

The North said the blast was a controlled explosion to prepare for construction of a hydro-electric power project and accused the South of spreading lies to divert attention from its own atomic revelations.

South Korea was recently forced to admit that its scientists carried out experiments to produce small amounts of enriched uranium and plutonium, both key ingredients in nuclear bombs.

The British diplomat said the region was usually out of bounds to all foreigners, including diplomats and aid agencies, and the British embassy had never before had access to the area.

Officials in Washington have played down speculation that a nuclear explosion took place, although other experts have said the North's explanation left many questions unanswered.

They have pointed to Pyongyang customarily pumping out propaganda about any major construction project, which it did not do on this occasion.

South Korean intelligence officials also questioned why the Stalinist state wanted to carry out a major earthmoving operation during the night given its power shortages.

A North Korean defector cited in the New York Times said it was near the main centre for launching ballistic missiles and theorised that a explosion of liquid nitrogen fuel could have happened with a missile on the launch pad.

"This was undoubtably an attempted launch," said Im Young Sun, who visited the site in the early 1990s as part of a military construction team.

Despite the swirling speculation, diplomats from the German embassy in Pyongyang said no special equipment was being taken by the German ambassador to measure radiation, despite it being available.

"As to my knowledge, the North Koreans did not exclude items to be taken with them," the diplomat said.

"I think what our ambassador and the others want to do is to have an eyesight of what is on the ground."

North Korea has courted controversy over its alleged nuclear weapons program, withdrawing from the Nuclear Non-Proliferation Treaty and re-starting its plutonium program.

Six-party talks aimed at resolving the North Korean issue appear to have stalled, with Pyongyang indicating it was unlikely to attend a fourth round of negotiations in Beijing scheduled by the end of this month.

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