The United States demanded Tuesday that North Korea come clean about its controversial highly enriched uranium program as the arch rivals ended landmark talks setting the pace for normalizing ties.

US Assistant Secretary of State Christopher Hill said North Korea reportedly made "massive purchases of expensive equipment" from Pakistan's once-dreaded A.Q. Khan illicit nuclear network to drive the highly enriched uranium program.

Enriched uranium is used as fuel for nuclear reactors, but highly enriched uranium can be used to make nuclear bombs.

"They need to come clean on it, explain what they have been doing, why they have been doing it, and ultimately they need to abandon it," Hill told a forum in New York on the sidelines of talks aimed at normalizing US-North Korean diplomatic relations after Pyongyang agreed to freeze a key nuclear facility in return for largely energy aid.

"I think we are owed a pretty clear answer why all these purchases were made and how far they have gotten into the process," Hill said amid a burgeoning controversy over the reliability of US intelligence on North Korea and whether Washington overstated Pyongyang's efforts to enrich uranium in 2002.

In October 2002, the United States accused Pyongyang of pursuing a covert program to produce highly enriched uranium, based on intelligence information.

At first, the North acknowledged the program -- leading to the scrapping of a 1994 deal to freeze Pyongyang's nuclear weapons drive -- but has since denied it.

US intelligence officials now say they have only moderate confidence it was still pursuing the goal, triggering speculations that Washington wanted to give Pyongyang a face-saving way to surrender its nuclear equipment.

Hill said he wanted North Korea to provide full details of the highly enriched uranium program as well as its plutonium activity at its Yongbyon nuclear facility during denuclearization talks in Beijing soon.

"We expect to do this in Beijing as early as next week -- begin the discussion on their (North Korea's) overall nuclear program, that is what needs to be abandoned pursuant to the agreement," he explained.

Under a February 13 accord, North Korea agreed to close and seal its Yongbyon facility within 60 days and admit UN nuclear inspectors in return for 50,000 tonnes of heavy fuel oil as a first step to eventual nuclear disarmament.

Even though the accord did not deal with the issue of plutonium that has already been produced by the facility, which some believe could be used to make up to a dozen nuclear bombs, Hill said all the sensitive nuclear material should be accounted for.

"Depending on which experts you talk to, estimates (are) that there are some 50 kilograms of plutonium that is already produced and will have to be accounted for," he said. "That plutonium we have reason to believe has been weaponized."

Hill and his North Korean counterpart Kim Kye-gwan meanwhile ended milestone talks Tuesday in a first step toward normalizing ties and cementing Pyongyang's commitment to scrapping its nuclear arms program.

It was the highest level meeting held in the United States between the two nuclear rivals since October 2000.

Hill said the discussions included devising criteria for North Korea to be removed from the state-sponsor-of-terrorism list and for scrapping longstanding US trade sanctions against the hardline communist regime.

Amid the talks, Washington came under pressure from an independent US commission to place long-standing human rights, humanitarian and refugee concerns squarely on the negotiating table with North Korea.

The human rights and humanitarian crises in North Korea deserve to be treated on a parallel track with security issues involving weapons of mass destruction and should not be marginalized, said Felice Gaer, chairwoman of the the bipartisan United States Commission on International Religious Freedom.

Gaer wanted Washington to tie future consideration of economic assistance or diplomatic recognition to reforms that would increase the freedoms of North Koreans and strengthen the security of the Korean Peninsula.

The commission is mandated by Congress to monitor abuse of freedom of religion or belief and related human rights around the world and to make recommendations to the President, State Department and Congress.

Hill said there were a number of "legal and political steps" that needed to be taken for eventual normalization of ties with North Korea, whose defiant atomic weapons test in October last year drew unprecedented UN sanctions.