By Arshad Mohammed and Sue Pleming

WASHINGTON, Dec 20 (Reuters) - The United States is still struggling to get North Korea to disclose its nuclear programs, a challenge in a society so tight-lipped that it would keep even clothing sizes secret, a U.S. official said on Thursday.

North Korea has promised to make a declaration by Dec. 31 as part of a wider deal to abandon its pursuit of nuclear weapons in exchange for economic and diplomatic benefits from the United States and others.

The senior official told Reuters that reflexively secretive North Korea was reluctant to detail its nuclear proliferation activities -- which it has steadfastly denied -- as well as what it regards as military secrets in its declaration.

"They have real weapons and so they should tell us what the weapons program looks like," said the official, who asked not to be identified because of the sensitivity of the negotiations.

"That is where you get into military secrets and, in a country that would keep a sweater size secret, you can imagine the difficulty in revealing military secrets," he added.

North Korea has published no economic statistics in the past several decades and severely limits its citizens' travel and contact with the outside world. Its leader, Kim Jong-il, has uttered only several words in public in his long career.

Sung Kim, a U.S. State Department official who is in Pyongyang to discuss North Korea's nuclear declaration, has reported progress in some areas but sticking points in others, the official said.

"It was a very mixed picture because we are not there yet with the declaration after one day. We made some progress but there are some sticking points remaining," the official said of Kim's first day of talks with officials from North Korea's General Directorate of Atomic Energy and its foreign ministry.

Kim has two more working days in North Korea to pursue the issue, which is widely seen in Washington as a key indicator of whether or not North Korea is committed to abandoning its nuclear weapons under a September 2005 multilateral deal.

Washington has already signaled that North Korea might not make the Dec. 31 deadline for the declaration.

The official was upbeat about North Korea's work to disable its Yongbyon nuclear facility, also due by Dec. 31, although he said this could slide into February for technical reasons. The foreign officials overseeing the disablement have chosen to go slowly on removing fuel rods for safety reasons.

"The disabling of the facilities is going ahead well. They have five of 11 tasks fully done and the others are under way. I think everyone involved with the disablement process is very satisfied," he said.

'NO ONE TURNS INTO A PUMPKIN'

The official showed Reuters before-and-after photographs of the disablement. One depicted a "fuel rod machine lathe" -- a piece of equipment that shapes plutonium fuel rods -- first with the machinery intact and then taken apart and removed.

"The timeline, of course, of Dec. 31 is notional. No one turns into a pumpkin on Dec. 31, but obviously we would like to get these 'phase two' tasks done at or around Dec. 31 so that we can move on to the next phase, which would be dismantlement and abandonment of the fissile material," he added.

Secretary of State Condoleezza Rice said on Thursday a lot was at stake and she wanted a "complete and accurate" declaration from North Korea.

"This is now a crucial step, because if we are to move forward ... on all of the benefits that would come to North Korea through the successful completion of this second phase, we really must have an accurate declaration," she said.

If North Korea provides the declaration and dismantles Yongbyon, the United States has dangled the possibility of removing it from the U.S. state sponsors of terrorism list.

The official said he expected Pyongyang to be taken off the list before U.S. President George W. Bush steps down in January 2009, assuming it qualified under U.S. law and met its denuclearization commitments.

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