BEIJING, Dec 6: North Korea must come clean on any efforts to enrich uranium with Pakistani help, a US envoy said on Thursday, while South Korea suggested the North may miss a year-end deadline for disclosing nuclear activities.

US Assistant Secretary of State Chris Hill visited North Korea this week to urge it to ‘disable’ its key Yongbyon nuclear complex and disclose all atomic activities by the end of December as part of a disablement deal.

Hill has said the North was moving to cripple the reactor and other units at Yongbyon so it will be difficult to restart.

But disagreement remains over what should appear in the tell-all declaration of nuclear activities Pyongyang has promised.

Before meeting Chinese diplomats to brief them on his Pyongyang trip, Hill indicated that one of the points of dispute was North Korea’s efforts to enrich uranium, a way of making nuclear material that does not rely on reactors.

“We’ve had a lot of discussions with them about uranium enrichment,” Hill told reporters, adding that the United States had “very good evidence” that North Korea had bought enrichment technology and had received assistance from Pakistan. We need them (North Korea) to step up and show some trust in us and in the process,” he added.

Under the Feb 13 agreement reached at six-party talks with the United States, South Korea, Japan, Russia and China, North Korea agreed to ‘disable’ Yongbyon and make the nuclear declaration in exchange for heavy fuel oil or equivalent aid.

US claims that Pyongyang pursued enrichment despite a 1994 disarmament pact were one of the developments that led North Korea to pull out of that pact in 2002 and restart Yongbyon, which can make plutonium usable for nuclear weapons. It tested a plutonium-based bomb in October last year.

The Bush administration, also distracted by Iran’s nuclear ambitions, has played down claims about how advanced North Korea was in enrichment. But Hill suggested that even if those efforts were fruitless or dormant, North Korea had to tell all.

“We want to be completely sure they don’t have any ongoing programme,” he said. “Being clear about what’s happened is also a means for us to build a future relationship.”

North Korea has denied to Hill obtaining gas centrifuges from Pakistan used to purify the type of uranium usable for reactor fuel or weapons, an expert on the dispute, Selig Harrison of the Woodrow Wilson Centre in Washington, told Reuters.