A published report says North Korean officials showed what they described as recently reprocessed plutonium to an unofficial American delegation who toured the Yongbyon nuclear facility last week.
The Washington Post says North Koreans told visiting U.S. nuclear experts and congressional aides that the material had not been placed in a nuclear device and that they were willing to "freeze" the weapons program to end the crisis.
Plutonium is a key ingredient of a nuclear bomb.
North Korea's official news agency (KCNA) said Saturday that Pyongyang had revealed what it described as its "nuclear deterrent" to the American delegation.
KCNA quoted a Foreign Ministry spokesman as saying that he hoped the delegation would help Washington change its view about Pyongyang's program.
However, one member of the delegation, Congressional aide to Republican Richard Lugar, chairman of the U.S. Senate Foreign Relations Committee, Keith Luse is cautioning against drawing any conclusiong about what the delegation saw, calling it "premature and speculation."
Mr. Luse spoke after arriving in South Korea Sunday. He is to brief South Korean officials Monday about the visit.
The aide was in the first group of foreign visitors to tour the nuclear complex since North Korea expelled U.N. inspectors a year ago. American officials described the trip as a private initiative, with no government involvement.
Delegation members have confirmed visiting the nuclear facility, but refuse to reveal any details until they brief U.S. officials.
U.S. officials have said they believe North Korea already has one or two nuclear bombs, and the ability to assemble several more.
China has been leading a diplomatic effort to reopen six party talks with North Korea, to try to persuade the Communist state to shut down its nuclear-development program.
Pyongyang is demanding written security guarantees, aid and policy concessions from Washington.
The United States says it will not provide such guarantees until Pyongyang abandons in a verifiable manner its quest for nuclear weapons.