Group visits N. Korea blast site
File picture of the region where the blast is believed to have gone off.
North Korea says cloud was the result of a deliberate demolition
Missile base near NK blast site
(CNN) -- Diplomats from eight countries are on their way to the site of a mysterious blast in North Korea that sparked speculation a nuclear test had taken place.
Japan's Kyodo news agency reported that the group would return late Thursday or Friday.
North Korea has said the blast was the result of a deliberate demolition of a mountain for a hydro-electric power project.
The mystery began when a 4-kilometer (2 miles) wide mushroom cloud was spotted on satellite images by South Korea's Yonhap agency.
The pictures were first seen three days after the blast took place.
U.S. Secretary of State Colin Powell told Reuters on Tuesday North Korea's explanation squared with Washington's view.
U.S. officials say they are still reviewing satellite imagery and other intelligence on the matter.
"We still don't know what it was, and we're not speculating," one administration official said.
Some outside analysts have speculated the explosion could have been at the Yongjo-ri Missile Base, a facility believed to house up to 36 NoDong missiles.
U.S. officials say there is no evidence that is true, though it cannot be completely ruled out.
According to data gathered by the Nuclear Threat Initiative (NTI), Yongjo-ri is a suspected site for North Korea's uranium enrichment program.
South Korea skeptical too
On Tuesday South Korea's defense minister said the country was seeking independent verification on the nature of the blast.
Yoon Kwang-woong said the South would use intelligence channels and satellite images to check on the source of the blast in a northern region of the North.
Hydroelectric experts in Seoul have questioned the North's explanation, saying the relatively small Huchang river in the area made it an unlikely and unfeasible site for a major hydro power plant, according to Reuters reports.
The nation's media have also raised questions, with the Chosun Ilbo newspaper quoting a North Korean defector familiar with the region who said the body of water in the area was not sufficient for a large power plant.
North Korea's official KCNA news agency said late Monday that reports of a large accidental explosion at the site or a nuclear test was a "preposterous smear campaign."
"Probably, plot-breeders might tell such a sheer lie, taken aback by blastings at construction sites of hydropower stations in the north of Korea," KCNA said.
CNN Radio, CNN National Security Correspondent David Ensor and Correspondent Sohn Jie-Ae contributed to this report.