David Slinn will head a group of diplomats from the embassies of Germany, Sweden, Russia, Poland, the Czech Republic, India and Mongolia, and may not be back until Friday.

The group will travel by plane for one hour and continue three hours by road before they reach the remote area close to the border with China, the diplomat told AFP from Pyongyang.

"It's not a scheduled flight," he said. "It's an old AM-25 propeller plane."

A senior British official had said the visit to the site was likely to take place either Tuesday or Wednesday, but the delay was due to practical problems of setting up the trip rather than North Korean stalling, he said.

He added that diplomats had been "surprised" at Pyongyang's relatively swift reaction to widespread foreign concern over the explosion, which reportedly triggered a mushroom cloud several kilometers (miles) in diameter.

North Korea has said the explosion, which happened last Thursday, was caused by a civil engineering project, and South Korean intelligence sources now believe the explanation might be true.

The North said the blast was a controlled explosion to prepare for construction of a hydro-electric power project in rugged Kimhyungjik county in Ryanggang province.

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