The revelations came amid reports that a huge explosion rocked North Korea's northern inland province of Ryanggang Thursday, triggering a mushroom cloud at least two miles in radius.

"The United States was known to have shown its keen interest in the explosion after spotting its traces by satellite," a source in Beijing told South Korea's Yonhap news agency, which carried the report.

South Korean officials Sunday downplayed the blast saying it was not likely linked to a nuclear test.

US officials would not immediately comment on the report or describe to AFP the information that prompted the intense debate, of which the White House has been made aware.

But one of them described it as serious enough to generate concern.

"It is well known and has been widely reported that North Korea's nuclear capability is a concern to us," one official said, speaking on condition of anonymity.

The New York Times reported Sunday that US intelligence picked up suspicious movement of materials around several locations deep inside North Korea that US analysts believe could eventually become nuclear test sites.

The suspicious activities involve one location where possible conventional components of a nuclear bomb were tested last year, according to the report.

But top analysts at US intelligence agencies differ on how to interpret the activities, primarily because they have not detected electrical cables leading into an underground test shaft, a telltale sign of preparations for a nuclear blast, the report said.

While confirming the internal discussions, another official described the debates as mainly "speculation," adding: "I can't substantiate the claims."

The office of South Korean President Roh Moo-Hyun said Sunday it was doubtful that a reported blast in North Korea was linked to a nuclear test.

"Our government information for now shows North Korea has not conducted any nuclear test," presidential spokesman Kim Jong-Min said.

Yonhap quoted a diplomatic source in Seoul as saying the blast triggered a mushroom cloud with a radius of 3.5 to four kilometers (2.4 miles).

"It is not clear yet whether the explosion is related to an intentional nuclear experiment or a simple accident," he was quoted as saying.

The explosion appeared to be stronger than an April 22 blast that killed more than 150 people and wounded some 1,300 others in Ryongchon near the western tip of North Korea's border with China.

The latest blast took place in Kimhyungjik county near the Chinese border on September 9, when North Korea marked the 56th anniversary of its founding, the report said.

A nuclear test by North Korea ahead of a US presidential election could deal a severe blow to President George W. Bush's strategy of diplomatic containment of the Stalinist state that has openly bragged about its nuclear ambitions.

The president has stated repeatedly that six-party talks involving all of North Korea's neighbors in addition to the United States is an effective tool for keeping these ambitions in check.

But he has been under fire from Democratic presidential candidate John Kerry, an advocate of direct negotiations with Pyongyang.

Officials do not discount the possibility of diplomatic brinkmanship on the part of the mercurial North ahead of the next round of six-party talks between the two Koreas, Japan, China, Russia and the United States set for later this month.

Pyongyang has hinted the talks could be jeopardized by South Korea's recent admission that its scientists carried out nuclear experiments.

The US Central intelligence Agency believes North Korea already has one or more nuclear weapons.