He was due in South Korea later on Friday.

Bosworth will also visit Japan and Russia next week but has no current plans to visit Pyongyang, the North Korean capital.

'Increased nuclear activity'

South Korean media on Thursday reported there was increased activity at North Korea's known nuclear test site, suggesting Pyongyang was gearing up for a new test.

After the UN Security Council condemned its April 5 rocket launch and tightened sanctions, the North announced it was quitting six-party nuclear disarmament talks and restarting a programme to make weapons-grade plutonium.

Last week it vowed to conduct a second nuclear test as well as ballistic missile tests unless the UN apologises for its actions.

The six-way talks group the two Koreas, the United States, China, Russia and Japan.

"Nothing would be expected from the US, which remains unchanged in its hostility toward its dialogue partner," the North said, denying suggestions that its actions were aimed at pushing Washington to hold direct talks.

"The measures... are aimed not to draw attention of someone and have dialogue with it but to defend the security of the country and the sovereignty of the nation."