A big explosion rocked a northern province of North Korea near the border with China last week, South Korea's Yonhap news agency has reported.
The blast is said to have happened last Thursday, as North Korea marked the 56th anniversary of its founding.
Yonhap quoted a diplomatic source in Seoul as saying a large mushroom cloud was spotted in Yanggang province.
South Korea's Unification Minister has reportedly played down the possibility that it was a nuclear weapons test.
Diplomatic officials in Washington are also quoted as saying the nature of the blast is unclear.
The diplomatic source in Seoul said the mushroom cloud, with a radius of up to four kms (2.5 miles), was spotted in Yanggang province's Kimhyungjik county.
Gee, I'm really glad we invaded Iraq which had no WMDs, and ignored North Korea, which now probably has nuclear weapons. Excitable statements from Dolly Downer about nuclear missiles hitting Sydney do not count as a policy.