For more than a year, Pyongyang boycotted six-party talks for nuclear disarmament after Macau regulators froze $24 million in its accounts in the Banco Delta Asia.

For the past 18 months, the US treasury department has been investigating the institution on charges that it had been accepting proceeds from the North's counterfeiting, drug-smuggling and money-laundering activities for years.

Last month, Pyongyang agreed to take the first step towards dismantling its weapons programme by shutting down the main reactor and re-admitting UN nuclear inspectors.

In return, it has been promised economic aid and political concessions.

But South Korea says the North has shown no signs of closing its nuclear complex, the source of its weapons-grade plutonium, since the pact was sealed in Beijing among six nations.

Song Min-soon, South Korea's foreign minister, said on Thursday: "There is no indication of a change in the operational condition of Yongbyon."

Some US officials earlier this week said they saw signs that North Korea was moving towards closing its nuclear complex but others thought it was still too early to tell.