SEOUL, Sept 13 (AFP) - South Korea said Monday it was investigating sketchy reports of a big explosion in North Korea last week but was unsure what triggered the blast near a secret underground military base.
However officials here and in Washington denied speculation that North Korea may have tested a nuclear device.
"We have received a report that there were signs of an explosion in North Korea," said Kim Hong-Ju, a spokesman for the Unification Ministry that handles relations with the communist North.
Another senior official said the government had "no clear information" about the nature of the explosion, saying officials were unsure whether it was "an accident or something else".
"All we can say so far is there are some signs that there might have been something and we are in the process of finding out what that was," the official said. "We don't really think that it was anything to do with anything that is nuclear."
South Korea's Yonhap news agency first reported the explosion on Sunday, quoting diplomatic sources as saying the blast triggered a mushroom cloud across a remote and mountainous area of North Korea on Thursday.
Yonhap said the explosion appeared to be stronger than an April 22 rail blast which killed more than 150 people near the border with China.
According to the news agency the explosion occurred in Kimhyungjik county in the remote province of Ryanggang near the Chinese border on the day when North Korea marked the 56th anniversary of its founding.
Some reports mentioned two explosions, one late Wednesday followed by another early on Thursday in the area -- which is home to an underground base for missiles and a suspected plant for enriching uranium, according to experts.
There was no word on the blast from the highly secretive North. It did not report the April rail accident in Ryongchon, near the western tip of North Korea's border with China, for three days.
The South Korean government rejected speculation that North Korea, which has often threatened to test a nuclear device, may have finally done so.
"Our government information for now shows North Korea has not conducted any nuclear test," presidential spokesman Kim Jong-Min said on Sunday.
US Secretary of State Colin Powell, also speaking Sunday, said he was unaware of any "nuclear event".
Some of the reports in the media here, which spoke of a mushroon cloud, also said the explosion triggered an earth tremor that measured 2.6 on the Richter scale. Officials declined to confirm the reports and South Korea's meteorological agency said it had no data indicating a nuclear test.
The reports came amid heightened diplomatic efforts to resolve the 23-month standoff over North Korea's nuclear weapons drive.
The top US envoy on North Korea, James Kelly, arrived in Beijing for a previously unpublicised visit just hours after the blast was reported.
Kelly's mission is part of efforts to persuade North Korea to take part in a new round of six-party talks on its nuclear programme that also saw Chinese and British officials in Pyongyang over the weekend.
North Korean officials knew nothing about the blast in their own country when they met a British government minister in Pyongyang, Britain's domestic Press Association news agency reported.
Bill Rammell, paying a three-day visit to Pyongyang, said it was only through his delegation that North Korean foreign ministry officials learned of the reported explosion, it said in a report from Pyongyang.
Pyongyang stunned the world in August 1998 by test-launching over Japan a Taepodong-1 missile with a range of up to 2,000 kilometers.