North Korea says it has revealed its "nuclear deterrent" to an unofficial delegation from the United States.
Yongbyon had been closed to outsiders for over a year
The US team confirmed they had seen the secret nuclear complex that Washington believes is being used to develop nuclear weapons.
They were the first group from outside North Korea to visit the Yongbyon facility since the North expelled UN inspectors at the end of 2002.
The visit came amid efforts to arrange new six-nation talks on North Korea.
Professor of international relations
Former head of Los Alamos Laboratory
Ex-member of US National Security Council
Members of the delegation, headed by Stanford University Professor Emeritus John Lewis, said they could not give details of their visit until their findings were reported to Washington.
But North Korea's official KCNA news agency quoted a foreign ministry spokesman as saying: "As everybody knows, the United States compelled the DPRK to build a nuclear deterrent.
"We showed this to Lewis and his party this time."
The BBC's Charles Scanlon, in Seoul, says the North seems intent on proving to the United States that it is not bluffing about its nuclear capabilities.
It has been frustrated by Washington's apparent lack of urgency and its refusal to negotiate directly, he adds.
The US and North Korea have been locked in a stand-off over the Communist state's nuclear weapons programme since UN inspectors were expelled in 2002.
Earlier this week, North Korea offered to freeze its nuclear programme in an apparent attempt to defuse the crisis.
But six-country talks aimed at resolving the issues may not resume until next month.
The Bush administration appeared to distance itself from the five-day visit, but our correspondent says US officials will be very interested in what the delegation saw.
Professor Lewis said the "full story" of the visit would be released "roughly in a week".
He said the delegates had been allow to visit all the places they had requested to view.
"What we saw in the DPRK were related to the whole of the issues, not just the nuclear issue," he said. "We were not there to negotiate, we were not there to be inspectors."
Fellow delegate Sig Hecker, who headed the Los Alamos National Laboratory from 1985 to 1997, said: "I feel a very deep obligation to first inform the US Government officials about our trip, what we saw and what we learned."
The delegation also met military, scientific and political figures.
In 1994, North Korea agreed to halt activities at Yongbyon, 90km (50 miles) north of the capital Pyongyang, under a deal with the United States.
But after that agreement broke down in late 2002, North Korea claimed to have finished reprocessing 8,000 spent fuel rods being stored at Yongbyon - enough to help it build up to six more nuclear weapons.
Foreign intelligence agencies have been sceptical about the claims, but have been unable to check them.
US President George W Bush's administration withdrew support for a congressional visit to North Korea in October because it said the timing was not appropriate.