Kim Sook, 55, a former director general of the foreign ministry's American affairs bureau, has replaced Chun Yung-Woo, the ministry said in a statement.

The new government of conservative President Lee Myung-Bak has replaced a series of senior officials in several ministries.

The change comes as the talks have reached a critical stage.

The United States, the two Koreas, China, Japan and Russia reached a deal last year which would grant North Korea energy aid and major diplomatic and security benefits in return for full denuclearisation.

But the talks have been stalled for months by a dispute over the communist state's nuclear declaration, which was promised by the end of last year.

Washington says the document should clear up suspicions about an alleged secret uranium enrichment programme and suspected proliferation to Syria. North Korea denies both charges.

Hopes of breaking the impasse emerged since last week when top US nuclear envoy Christopher Hill and his North Korean counterpart Kim Kye-Gwan met in Singapore to debate the form of the declaration.

US President George W. Bush has accepted the tentative deal reached in Singapore, according to White House spokeswoman Dana Perino on Monday.

Media reports say the North would "acknowledge" concerns about uranium and proliferation in a secret side-agreement with the US. The main declaration would refer only to the acknowledged plutonium-based weapons operation.

"I will do my best to make contributions to laying a stepping stone for peace and stability on the Korean peninsula through the resolution of the nuclear crisis," Kim told Yonhap news agency.

After joining the foreign service in 1978, Kim has spent most of his career handling North American affairs. He served as consul-general in Toronto before heading the American affairs bureau from 2004 to 2006.

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