North Korea, which last month defied the international community by test-firing missiles, could now be preparing its first test of a nuclear bomb, US media cited US officials as saying.
Analysts said yesterday North Korea could be trying an even more extreme form of saber-rattling to force the international community into making concessions.
However, South Korea said it has no clear evidence suggesting that North Korea is preparing for a nuclear test.
"I haven't heard that we have confirmed clear evidence that North Korea is pursuing a nuclear test," Unification Minister Lee Jong-seok told lawmakers at a committee hearing.
Lee said there is frequent speculation about the North's nuclear program, but that not all of it turns out to be true.
ABC News on Thursday quoted a senior military official as saying a US intelligence agency had observed suspicious vehicle movement at a suspected North Korean test site, including the unloading of reels of cable outside an underground facility.
Such cables are connected to outside monitoring equipment and could be a possible sign of an upcoming test.
A senior US State Department official, who was not identified, told the network: "It is the view of the intelligence community that a test is a real possibility."
The July 5 missile tests were widely seen as less a display of military muscle than a repeat of Pyongyang's often-used tactic to grab international attention but whose key aim -- to win direct talks with the US -- failed.
"Many of the experts ... have been concerned about the possible options on the part of North Korea, including nuclear tests or other forms of military provocation," said Kim Sung-han, head of North American studies at South Korea's Institute of Foreign Affairs and National Security.
Pyongyang accuses Washington of trying to topple its government through a crackdown on its finances. It wants this ended before it will return to international talks to end its nuclear weapons program.
The US refuses to end the crackdown or to hold direct talks with the North outside the six-country nuclear negotiations.
Asked about the media reports, a senior US official said: "We have no new evidence to support that."
North Korea has been working on nuclear weapons for years. A June report from the Washington-based Institute for Science and International Security said the North had enough radioactive material to build between four and 13 bombs.
But officials say no one knows for sure whether it has actually built a nuclear weapon. The North declared itself a nuclear weapons power in February last year, without testing.