North Korea is likely to be producing nuclear bombs even as it conducts negotiations with the United States and four other countries on ending its weapons programs, the senior U.S. official responsible for those talks told Congress yesterday.

"Time is certainly a valid factor in this," said James A. Kelly, the assistant secretary of state for East Asian affairs, during testimony before the Senate Foreign Relations Committee. "We don't know the details, but it's quite possible that North Korea is proceeding along, developing additional fissionable material and possibly additional nuclear weapons."

Although North Korea has asserted that it has produced weapons-grade plutonium since the crisis over Pyongyang's nuclear programs began 20 months ago -- and though U.S. intelligence analysts broadly believe that the number of nuclear weapons held by North Korea has increased from two to at least eight during this period -- it is highly unusual for a senior administration official to concede publicly that North Korea's stockpile may be growing.

After four negotiating sessions with North Korea and its neighbors since April 2003, Kelly said, it "is clear we are still far from agreement." The first round included China and later expanded to involve South Korea, Japan and Russia.

Democrats on the committee scolded the administration for waiting too long to present North Korea with a detailed proposal for ending the crisis. At the most recent six-nation talks, held in Beijing last month, the administration proposed that once North Korea declares it would end its programs, U.S. allies such as South Korea could provide immediate energy assistance.

North Korea then would have three months to disclose its programs and have its claims verified by U.S. intelligence. After that, the United States would join in providing Pyongyang with written security assurances and participate in a process that might ultimately result in the normalization of relations.

"The bottom line is that we now confront a much more dangerous adversary than we did in 2001," said Sen. Joseph R. Biden Jr. (Del.), the ranking Democrat on the panel. He accused the administration of adopting a policy of "benign neglect" even after learning that Pyongyang had a clandestine nuclear effort, and then taking "more than two years to resolve its internal divisions and settle on an approach for dealing with North Korea."

Under questioning, Kelly made it clear that improving relations with North Korea would take much more than the dismantling of its nuclear programs. In particular, he said, North Korea would need to improve its human rights record.

"We're not looking to bribe North Korea to end its nuclear weapons state," Kelly said. "We see this as a very important objective, but then we have made clear that normalization of our relations would have to follow these other important issues. And human rights is co-equal in importance, perhaps even more important, than conventional forces, chemical weapons, ballistic missiles, matters of that sort."

In response to the administration's proposal, North Korea has demanded immediate assistance from the United States once it freezes its programs. Kelly said the administration is still studying the North Korean proposal, which he called vague.

He told lawmakers that the administration does not consider the security assurances a "reward" or a benefit that could be claimed by North Korea as a U.S. concession.