The White House has warned North Korea against moving to manufacture weapons-grade plutonium by reprocessing nuclear fuel rods.

Such a step would be a "provocative action... intended to intimidate and blackmail the international community", said spokesman Ari Fleischer.

About 8,000 nuclear fuel rods are said to be stored at Yongbyon

The warning came after reports that US spy satellites had detected lorries taking on cargo at the Yongbyon nuclear power plant.

The plant, about 80 kilometres (50 miles) north of Pyongyang, houses stockpiles of nuclear fuel rods.

Tension between the two states escalated late last year after the United States accused North Korea of abandoning a 1994 pact to forsake its nuclear ambitions.

North Korea's nuclear programme has come under intense international scrutiny since it expelled UN weapons inspectors in December.

Experts say that if North Korea really has begun moving the 8,000 fuel rods stored at Yongbyon, it could either be attempting to hide them or moving them to a reprocessing plant to produce bomb-grade plutonium.

Other commentators have suggested that North Korea is well aware the plant is under surveillance and it is trying to improve its bargaining position with the leverage afforded by the nuclear threat.

'Non-compliance'

North Korea pulled out of the global Nuclear Non-proliferation Treaty on 10 January.

CRISIS CHRONOLOGY

16 Oct: US announces that N Korea has acknowledged secret nuclear programme 14 Nov: Oil shipments to N Korea halted 22 Dec: N Korea removes monitoring devices at Yongbyon nuclear plant 31 Dec: UN nuclear inspectors forced to leave North Korea 10 Jan: N Korea pulls out of anti-nuclear treaty 24 Jan: North-South talks end without making progress 28 Jan: President Bush urges the "oppressive" N Korean regime to give up its nuclear ambitions 30 Jan: N Korea responds with allegations of American hypocrisy and aggression 31 Jan: White House warns N Korea over suspected nuclear activity

Timeline of tensions

It insists it is the target of US "aggression" and that by halting oil shipments last year, the US has forced it to restart nuclear power stations.

Also on Friday, Mohamed ElBaradei, head of the International Atomic Energy Agency, said the North Korean nuclear issue should be referred to the UN Security Council.

He said the 35 agency members had tentatively agreed to meet on 12 February to begin such proceedings.

The board could then declare North Korea in "non-compliance" of international obligations on nuclear weapons.

This would be a sensitive issue as it could then pave the way for the Security Council to impose sanctions on an already isolated and poverty-stricken country.

But, Mr ElBaradei said, "North Korea is in non-compliance, so we need to get the board to certify that conclusion".