"They need to come clean on it ... explain why they are doing it," US Assistant Secretary of State Christopher Hill said.

Hill was speaking at a forum for the New York-based Japan Society on the sidelines of negotiations aimed at normalizing US-North Korean diplomatic relations after Pyongyang agreed to freeze a key nuclear facility in return for largely energy aid.

Hill met Monday with his North Korean counterpart Kim Kye-gwan Monday before hosting his visitor to dinner at New York's grand Waldorf Astoria Hotel. The two were to resume talks Tuesday.

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

The talks coincide with a burgeoning controversy in Washington over the reliability of US intelligence on North Korea and whether Washington overstated Pyongyang's efforts to enrich uranium in 2002.

US intelligence officials defended their work over the weekend but said that while they had high confidence Pyongyang tried to enrich uranium in 2002, they had only moderate confidence it was still pursuing the goal.

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