U.S. intelligence has proof that North Korean nuclear materials ended up in Libya (search), the top U.S. envoy on the standoff over Pyongyang's nuclear program said Thursday, as he expressed concern about the potential for more proliferation by the isolated communist nation.

Christopher Hill, outgoing U.S. ambassador to South Korea (search), also told The Associated Press he would travel to China soon to push for a resumption of international disarmament talks with North Korea -- his first trip after moving next week to Washington to become the U.S. assistant secretary of state for East Asian and Pacific affairs.

Hill said there was "physical evidence that the material that arrived in Libya had started its journey" in North Korea, speaking in his office at the U.S. Embassy in Seoul.

Although the material passed through the nuclear black market network run by Pakistan's A.Q. Khan (search), Hill questioned how the North Koreans could have been unaware of its eventual buyer. But he wouldn't go so far as to say U.S. intelligence had proof of direct contact or payments between Libya and the North.

"This is not a regime that gives you a lot of confidence that they know where to draw the line," he said of the risk of future proliferation by North Korea.

Attempts to reach an official in Libya on Thursday for comment were not successful.

Asked about Hill's comments, Melissa Fleming, a spokeswoman for the International Atomic Energy Agency in Vienna, Austria, said: "It's a possibility, but it's difficult for us to verify because we no longer have any inspectors there" in North Korea.

Hill said a recent visit to China by Kang Sok Ju, North Korea's first vice foreign minister who has been directly involved in the nuclear issue, could be key in efforts to bring Pyongyang back to nuclear disarmament talks that have been on hold since June. Hill said, however, that Beijing had not released details of Kang's meetings.

Hill said the United States has seen no indication that North Korea will return to the discussions.

On Feb. 10, the North declared it would indefinitely boycott the talks -- which include China, Japan, Russia, the United States and the two Koreas -- and claimed to have built nuclear weapons. Since then, North Korea has vowed to stay away from the negotiations until Secretary of State Condoleezza Rice apologizes for labeling North Korea one of the world's "outposts of tyranny."

Rice has dismissed the demands for an apology, and Hill on Thursday questioned the North's seriousness in making such a plea.

"There's a real mismatch between the enormity of the issue -- that is, nuclear weapons -- and the banality of their request," Hill said.

Some of the other countries in the arms talks have called on the United States to offer something to the North to lure it back to the negotiations.

"Our wish is for the United States to play a more active role," Rhree Bong-jo, South Korea's deputy unification minister, told reporters in Washington on Wednesday, according to South Korean reports. "It is important that all nations involved (in the nuclear talks) show elasticity and a flexible attitude in resolving the problem."

But Hill reaffirmed Washington had no plan to sweeten its current proposals, including security guarantees and aid in exchange for the complete dismantling of North Korea's atomic weapons program. Any change to that offer could only come if the North returns to the talks, he said.

"If you go that route, you're just encouraging that kind of behavior," Hill said. "We're not asking them to come out with their hands up, we're just asking them to come to the table."

He also dismissed North Korea's contention that it needs nuclear weapons to deter a U.S. attack.

"To believe that they need them for deterrence is to believe we're about to invade North Korea, which just isn't true," Hill said.

Instead, he said the North's nuclear aspirations had more to do with its image and provided a way for it to look past its other problems, such as failing to provide enough food to its own people. "They have something in their mind about being a great power. This is a shortcut," he said.

Hill maintained that U.S. officials weren't yet discussing sanctions or other steps beyond the six-nation nuclear talks. While not specifying any deadline for the North to return, Hill hinted Washington's patience will eventually run out.

"This is not an issue we can walk away from," he said.