The US cutter Sequoia left Shanghai for Japanese waters, where it will be engaged by the Japanese coast guard in a mock operation later on Monday.

"As of today China and South Korea have not participated in the drill," said Akihisa Watanabe, an official at the Japan Coast Guard's guard and rescue division.

"The statements of both countries explaining the reason for the cancellation say that unexpected other tasks just came up," he said.

But diplomats in Beijing said China was concerned about upsetting its ally North Korea, which is widely suspected of proliferating weapons and is boycotting six-nation talks on ending its nuclear drive.

China and South Korea both refused to take part in another US-backed drill in October 2004 in Japanese waters, which the United States had called a warning to North Korea.

Japan, which organized the latest drill, said it was not part of the Proliferation Security Initiative which covered the 2004 exercises.

In a last-ditch bid to persuade neighboring countries to take part, the Japan Coast Guard revised the description of the drill to say it would simulate capturing a ship "engaged in smuggling goods and people."

The original statement had said the ship would be "carrying weapons of mass destruction."

China has hosted the six-nation nuclear talks, which have been at a standstill since November as North Korea demands the lifting of US financial sanctions over counterfeiting and money-laundering.

South Korea has increasingly clashed with the US line on North Korea and favors engaging its estranged neighbor through trade and aid.

The South Korean coast guard said it objected to Japan's reference to weapons of mass destruction.

"The Korea Coast Guard raised the issue and asked for a delay of the drill, but the request was not accepted. So it decided not to participate," it said in a statement.

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