Progress on disabling North Korea's main nuclear facility was on schedule to be completed by year's end, but the toughest step — getting the North to give up existing weapons material — was still to come, the chief U.S. negotiator said Tuesday.

U.S. Assistant Secretary of State Christopher Hill said six-party negotiations to end the North's nuclear programs were proceeding step by step, but that persuading the country "to give up all its weapons in one fell swoop is probably not going to happen."

North Korea in February agreed to abandon its nuclear ambitions in return for 1 million tons of oil aid and political concessions, as part of long running negotiations with the United States, China, Japan, Russia and South Korea.

Under the initial phase of the deal, North Korea in July shut down its Yongbyon nuclear complex, which is believed to have produced enough plutonium for perhaps more than a dozen bombs — including the device North Korea detonated a year ago to prove its long-suspected nuclear capability.

A U.S. team of nuclear experts has been in North Korea since Thursday to map out a disablement plan, and Hill said Tuesday during a trip to Australia they were making progress.

"So far so good," Hill told Australian Broadcasting Corp. radio. "I mean, the North Koreans have been pretty good about granting access. It's been pretty collegial, if I can use that expression to talk about the North Koreans."

"The hope is by the end of this year we will have a disabled nuclear facility, we will have a full catalog of what all their facilities are," Hill said.

"And then, next year, we will try to get to the last step, which is to get them to give up, to abandon, the 50 kilos of plutonium that they have already produced," he said. "That will be the tough part."

In an interview Friday with CNN, Hill estimated that North Korea possesses "about 110 pounds" (50 kilograms) of reprocessed plutonium, which he said the country must give up.

In remarks to the Sydney Institute think tank earlier Tuesday, Hill said he believed the communist regime would terminate by the end of this year any programs it has on enriching uranium.

"I think by the end of the year we will have good reason to believe that whatever uranium enrichment program they have going, they will not have going by the end of the year," he said.

Uranium enrichment, a process needed to prepare materials for atomic weapons, has long been suspected as being part of the North's programs.

Meanwhile, working-level officials from the two Koreas, the United States and their regional partners were expected to meet as early as next weekend to discuss details of the aid-for-disarmament deal, South Koreas Yonhap news agency reported, citing unnamed government officials.