North Korea has begun rebuilding a nuclear reactor it recently knocked down as part of a disarmament agreement, in an apparent reprisal for delays in the delivery of Washington's end of the deal.

South Korea confirmed reports yesterday that reconstruction had begun at the Yongbyon nuclear complex, where the North Koreans are believed to have produced plutonium for about six bombs.

Washington has insisted that North Korea introduce transparency measures so that the dismantling of its nuclear programme can be verified.

Critics say that those measures were not part of the deal, brokered by a six-nation group, which exchanged disarmament for aid.

A South Korean foreign ministry spokesman, Moon Tae-young, said: "Our government expresses serious concern because this goes against the movement toward denuclearising North Korea and damages the six-nation process."