North Korea has told a visiting US congressmen that it already has nuclear weapons and intends to build more.
Mr Weldon has just returned from a three-day trip to Pyongyang
Republican Curt Weldon said after a three day visit to Pyongyang that officials from the communist state had "admitted to having nuclear capability" as well as trying to "expand their nuclear production programme."
Meanwhile the US has turned down a North Korean request for bilateral talks, insisting that the only way forward is through multilateral negotiations.
Pyongyang's request was passed to US President George Bush by the Chinese president, Hu Jintao, when they met at the Group of Eight (G8) summit in France on Sunday.
Tension over the continuing nuclear standoff is escalating, with the US getting increasingly frustrated by North Korea's habit of taking a different line in public to what it admits in private, correspondents say.
South Korean President Roh Moo-Hyun said on Monday that despite Mr Weldon's comments, there was still no "clear proof" that North Korea possessed nuclear capabilities.
In a speech marking his first 100 days in office, Mr Roh said "various options" needed to be discussed in handling the crisis sparked by the North's nuclear ambitions.
US Deputy Defence Secretary Paul Wolfowitz also spoke about the continuing nuclear standoff on Monday, promising swift retaliation to any North Korean aggression.
"Our response to aggression will be united, immediate, and devastatingly effective," he said.
Mr Wolfowitz warned of 'devastating' action against N Korean aggression
Speaking during a brief visit to South Korea, en route to meetings in Japan, Mr Wolfowitz said Pyongyang should stop the "enormous diversion of its limited resources" to its military and concentrate instead on the welfare of its impoverished people.
Mr Wolfowitz also discussed a controversial plan to overhaul US troop deployment in South Korea.
The authorities in Seoul have opposed any suggestion to redeploy the 14,000 frontline troops from the border region, a move which would take them out of range of a North Korean missile and rocket attack.
According to one US defence official, the troop changes would be aimed at ensuring that the US and South Korea would be able react swiftly in the event of a war with the North.
The troops would be able to begin "taking down" North Korea's heavily defended frontline, in the Demilitarised Zone (DMZ) within the first hour of hostilities, the official told Reuters on condition of anonymity.
"While we can't completely compensate for the fact that North Korea has so much stuff right up forward on the DMZ, we could begin taking it down from the first hour of the war and that would make a big difference," the official said.
North Korea has repeatedly accused Washington of planning to invade the country, and America has not ruled out military action, although President Bush has said he would prefer a diplomatic solution.
'Admission' of weapons
While not travelling as envoys of the US administration, Mr Weldon's team was the first official American group to visit the reclusive state since the nuclear standoff began in October.
Mr Weldon told a news conference in Seoul on Monday that North Korea had admitted not only to having nuclear weapons but intending to build more.
"They admitted to having just about completed the reprocessing of 8,000 rods," Mr Weldon said, referring to the fuel rods from atomic reactors which experts say could be used to make several nuclear bombs.
The North Koreans said they were developing their weapons as "a response to what they saw happened in Iraq, with the
US removing Saddam Hussein from power," Mr Weldon said.
Mr Weldon's comments echo those of US officials following talks with North Korean officials in Beijing in April.
American delegates said then that North Korea had admitted to developing nuclear weapons, but would be prepared to give them up in return for economic and diplomatic concessions.
But Washington has repeatedly stated that it would not give in to what it called North Korean blackmail.