Beijing ? Video footage of the area where North Korea said a huge explosion occurred showed dozens of workers swarming around a dusty construction site resembling a large dam project, while a foreign diplomat who visited the site said Friday he found no sign the blast was nuclear.

South Korea, meanwhile, said a mushroom-shaped plume thought to be from the Sept. 9 blast was 60 miles from the site where North Korea said it occurred and might have been a natural cloud formation.

Diplomats from seven countries were flown Thursday by the secretive communist state to its remote northeast, near the border with China, to verify claims that the explosion was part of work on a hydroelectric dam — not a test of its contentious nuclear program.

“One thing is entirely clear: This was not a nuclear explosion that happened at this site,” Sweden’s Ambassador to North Korea, Paul Beijer, said by phone from North Korea’s capital, Pyongyang. “This is a site where thousands of people are working on dam building.”

Concern was sparked when South Korea reported days after the blast that a mushroom cloud more than two miles wide had been spotted on satellite.

Independent video of the construction site was obtained by Associated Press Television News in the North Korean capital of Pyongyang, hours after the ambassadors returned from their visit.

The video, apparently shot from a point high above the valley floor, showed a building complex intact near a place where rock had been blasted away, with scores of workers moving around.

A deep excavation with large pools of water and wooden shelters could be seen across the valley, apparently where the dam is intended to rise, and a North Korean official was shown pointing out a big billboard illustrating the completed project.

The size of the cloud and the timing of the blast, which coincided with the 56th anniversary of North Korea’s founding, fed speculation by South Korean media that it was a nuclear test.