By Louis Charbonneau

UNITED NATIONS, Sept 11 (Reuters) - U.N. Secretary-General Ban Ki-moon said on Thursday he was worried about North Korean plans to reassemble nuclear facilities and urged Pyongyang to keep its promise to dismantle its nuclear weapons program.

Last week U.S. officials said North Korea had taken out of storage some mothballed equipment at its Yongbyon nuclear complex in what appeared a sign of displeasure over talks on ending its atomic programs.

Ban, who is South Korean, told reporters he was "concerned deeply by (North Korea's) decision to go back to reassembling the nuclear facilities.

"They must commit to their agreement among the six-party talks for the early realization of the denuclearization process," he said.

Pyongyang, which tested a nuclear bomb in 2006, began disabling the Soviet-era reactor and other facilities at Yongbyon in November 2007 as a step toward dismantling them completely. The move came in exchange for economic aid and political concessions including removal from a U.S. terrorism blacklist.

But Pyongyang announced on Aug. 26 it would stop disabling Yongbyon and accused Washington of violating the denuclearization deal negotiated in six-party talks that included the two Koreas, China, Japan, Russia and the United States.

Pyongyang said it did so because Washington had failed to drop it from its list of state sponsors of terrorism. The United States countered that North Korea must first agree on a system to verify its disclosures about its nuclear programs.

Reacting to reports that North Korean leader Kim Jong-il was recovering from a stroke, Ban said he hoped this would not affect Pyongyang's commitment.

"I only hope that any situation happening ... should not affect negatively what has been going on in terms of denuclearization process ... of the Korean peninsula as agreed by the six parties," he said.

Analysts say the process of ridding North Korea of atomic weapons would almost certainly be made more complicated in the event of a leadership struggle in the communist state.

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