An unofficial U.S. delegation says it gained access to North Korea's Yongbyon nuclear complex, but the delegates refused to provide any detail for the moment on what they saw there.

The team of U.S. academics and congressional staff members passed through Beijing on Saturday after a five-day stay in North Korea. Delegation member Sig Hecker, a nuclear scientist, told reporters the group was taken to the Yongbyon facility.

"We visited a number of places, including Yongbyon," he said.

The facility north of Pyongyang is where North Korean officials say they have been reprocessing thousands of spent nuclear fuel rods in order to build nuclear weapons, and is a key element in the current North Korean nuclear crisis. The U.S. team was the first group of outsiders to visit the complex since U.N. inspectors were expelled from North Korea a year ago.

However, neither Dr. Hecker, who works for the Los Alamos National Laboratory in the United States, nor any other members of the delegation would give details of what they observed at the complex. They said they first had to report to their respective headquarters in the United States.

One member, Professor John Lewis of Stanford University's Center for International Security and Cooperation, said the North Korean officials who hosted the delegation seemed cooperative.

"We sent them a list of all our requests," said John Lewis. "They honored all of them and indeed when we made some additional ones, they honored all of those."

The delegation met with North Korean economic, scientific, military, and foreign ministry officials. Again, there were no details of what was discussed.

Although the delegation included two U.S. Senate staff members, Professor Lewis emphasized that the trip was strictly unofficial.

"We were not there to negotiate," he said. "We were not there as inspectors."

Members did say some of them received permission from U.S. government entities, including the White House, to travel to North Korea, with which the United States does not have diplomatic relations.

The delegation's visit came as the United States, China, and others work to arrange a new round of six-party negotiations to resolve the nuclear crisis. The dispute emerged more than a year ago, when North Korea admitted it had restarted its nuclear weapons program, in violation of several international agreements.

The United States and its allies are demanding that North Korea dismantle the program in a verifiable manner.