South Korea's foreign minister says Seoul and Washington have no evidence to confirm that North Korea has finished reprocessing spent fuel rods for nuclear weapons.
The North is thought to have stored 8,000 spent fuel rods
"South Korea and the United States are trying to secure data
[on the North's reprocessing] through various sources, but
evidence has not emerged yet," Yoon Young Kwan said.
He expressed doubt about Pyongyang's reported claim that it completed reprocessing rods that could produce enough plutonium for several atom bombs last month.
Countries in the region have expressed renewed concern about the dispute over the North's nuclear programme.
"It is not desirable for North Korea to escalate the nuclear
situation. North Korea should be aware that if they keep it up,
it will only lead to one thing - isolation in the international
community," Japanese government spokesman Yasuo Fukuda was quoted as saying.
Australian Prime Minister John Howard expressed concern on Sunday that tensions over Pyongyang's nuclear programme could deteriorate into outright hostilities.
Mr Howard, speaking ahead of a visit to the Philippines, Japan and South Korea, said North Korea was a rogue state and "clearly in breach of international obligations".
Since international monitors were expelled from North Korea six months ago, it has been hard for the outside world to know exactly what is happening at the country's nuclear facilities.
American satellites and spy planes have been keeping watch, but US officials concede their intelligence is imperfect.
But in recent days there have been a series of reports that the North has reprocessed spent nuclear fuel - the final step in the creation of weapons-grade plutonium for atom bombs.
The South Korean news agency, Yonhap, says North Korean diplomats told US State Department officials in New York that the process was completed on 30 June.
Japanese newspapers earlier quoted US intelligence as saying that a by-product of reprocessing, known as Krypton 85, had been detected in the air near the North's nuclear plants.
Mr Howard said diplomatic pressure was needed to return North Korea to the international community.
"We're dealing with a country that is not operating, as it were, within the square. This is a very dangerous situation," Mr Howard told reporters last week.
South Korea's intelligence agency believes reprocessing has begun, but the BBC's Charles Scanlon in Seoul says there is still confusion and ambiguity.