BEIJING - The North Korean government has disavowed an earlier commitment to negotiate a step-by-step elimination of its nuclear weapons program with the Bush administration but may freeze the production of nuclear bombs under strict conditions, said an American expert on North Korea who completed a visit there this weekend.

The expert, Selig S. Harrison of the Center for International Policy in Washington, said in an interview that he was informed by several top-ranking North Korean leaders that the United States must pledge to respect the country's sovereignty and territorial integrity before any freeze could be discussed. The Bush administration has rejected preconditions for resuming negotiations.

"We have lost the opportunity to negotiate a step-by-step agreement that would lead to the eventual dismantling of their nuclear program," Harrison said in Beijing after returning from Pyongyang, North Korea's capital. "They are no longer willing to discuss that possibility."

Harrison has been critical of the Bush administration for failing to engage in direct negotiations with the North Koreans. He has had a rare high-level access to the North Korean leadership.

On his most recent visit, he said, he met with Kim Yong Nam, North Korea's second-highest official and the nominal head of state; Kang Sok Ju and Kim Gye Gwan, senior Foreign Ministry officials who oversee talks on the nuclear program, and Gen. Ri Chan Bok, who is in charge of North Korean forces at the truce village of Panmunjom at the border.

Although the North Koreans are willing to return to six-nation nuclear talks that have taken place under Chinese auspices, they are demanding that the United States first offer an apology for Secretary of State Condoleezza Rice's comment during congressional hearings that North Korea was an "outpost of tyranny," Harrison said.

Harrison said this constituted a "major policy shift" that had taken place since his last visit to Pyongyang, a year ago. He said he attributed the shift to hard-line military elements that have exerted more control in recent months.