North Korea has agreed to demolish the cooling tower at its main atomic complex soon after it makes a declaration of its nuclear programme, a South Korean official said.

The cooling tower at North Korea's Yongbyon nuclear complex, about 60 miles (100 km) north of Pyongyang, has been the focus of US surveillance in recent years for indications that the North was operating a plutonium-producing reactor there.

"It will come on the heels of the declaration," Kim Sook, South Korea's chief envoy to six-country talks on ending the North's nuclear programme, said when asked about the demolition of the cooling tower.

Kim spoke after returning from Washington, where he held talks with US and Japanese nuclear negotiators.

Once the declaration is made, the United States is expected to drop North Korea from its list of state sponsors of terrorism and to end sanctions imposed under the US Trading With the Enemy Act.

The moves are part of a broader deal under which North Korea agreed with the South, the United States, Japan, Russia and China to abandon all nuclear programmes in exchange for economic and diplomatic incentives.

The cooling tower is part of a five-megawatt atomic reactor that produced an estimated 50 kg (110 lb) of plutonium. Experts say that is enough for about eight nuclear bombs.

North Korea set off a nuclear device in October 2006, a test it heralded as proof that it was a nuclear state but the success of which was questioned by officials in the South.