Britain has demanded an immediate explanation from North Korea after reports of a blast that left a mushroom shaped cloud there last week.
South Korean media say the explosion took place in Yanggang Province, near China's border, as the Stalinist state celebrated its National Day.
UK Foreign Office minister Bill Rammell - in North Korea for talks - said he had asked for "an urgent explanation".
Western officials have downplayed suggestions it was a nuclear test.
First visit
Mr Rammell stressed he only had provisional reports of Thursday's explosion.
"I think it is important we get to the bottom of exactly what happened," he said.
"I have asked for an urgent explanation from the Ministry of Foreign Affairs and I will certainly be raising it with the Foreign Minister tomorrow morning."
North Korea has claimed to have nuclear weapons and to have been working on developing its arsenal.
But the outside world has been unable to verify those claims.
Mr Rammell is the first British minister to visit North Korea and will participate in three days of talks.
He is being accompanied by the Foreign Office's chief human rights expert, Jon Benjamin, who he hopes will make a return visit to Pyongyang at a later date.
The UK refused all previous invitations until the communist state agreed to discuss human rights.
Mr Rammell opened talks on the state's human rights record, describing recent allegations as "the worst in the world".
North Korea's vice foreign minister Kung Sok Ung has promised Mr Rammell another meeting with other officials on Monday.
Mr Rammell told the BBC he would also be pressing the North to rejoin the "six-party" international talks - involving both Koreas, the US, China, Russia and Japan - over its nuclear weapons.
"I think this is going to be a long haul and it's about trying to pull North Korea back from complete isolation," he added.
The highly secretive country has threatened to pull out of the talks because of perceived American hostility.
'Realistic expectations'
Mr Rammell will use Libya as an example of a nuclear state that has been welcomed back by the international community.
He will also highlight the advantages to North Korea if it opened up including international aid and that help with energy supplies could follow.
"North Korea has a key choice. It can engage in this process and get rid of what it has got and promise not to develop anything further.
"Then all sorts of positives can come its way. Isolation is the alternative route."
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