Siegfried Hecker, former head of the US laboratory which developed the atom bomb, "said he thought it was plutonium," a US official told Knight-Ridder newspapers on condition of anonymity.

Without sophisticated measuring equipment it was impossible for Hecker to conclusively identify the substance as plutonium, a key ingredient of nuclear weapons, the officials said.

Hecker, former head of the Los Alamos National Laboratory in New Mexico, gave the Senate Foreign Relations committee a private account of his visit on Tuesday, congressional sources said but declined to give details of his testimony.

The US scientist has also briefed senior US foreign policy officials on the visit and is due to meet Senators again on Wednesday, for a public rundown of the visit earlier this month, his presentation stripped of sensitive intelligence information.

Hecker was part of two non-governmental delegations which visited the Yongbyon nuclear complex, 90 kilometres (50 miles) north of Pyongyang, where the communist regime has said it is processing weapons grade plutonium.

North Korea said after the trip that it showed its "nuclear deterrent" to the US delegation but it was unclear what exactly was meant by the comments.

The United States and North Korea have been locked in a nuclear showdown since Washington accused Pyongyang in October 2002 of embarking on a banned program to enrich uranium for nuclear weapons, in violation of a 1994 anti-nuclear pact.

Pyongyang offered on January 12 to freeze its nuclear reactors producing weapons grade plutonium if compensated by Washington -- an exchange the Bush administration has been loath to endorse.

North Korea has previously admitted that it fired up the reactors at Yongbyon but it is not clear if it has reprocessed 8,000 spent fuel rods frozen after the 1994 anti-nuclear deal.

US intelligence experts believe that stock could provide Pyongyang with the ingredients for up to six nuclear bombs. The CIA says the Stalinist state may have already possessed one or two crude atomic weapons.

Washington halted its fuel oil shipments to the energy-strapped country in late 2002 soon after accusing North Korea of running a secret uranium-enrichment program violating the 1994 deal to mothball the Yongbyon plant.

In retaliation, Pyongyang said it reactivated the Yongbyon complex.

It has since claimed that it reprocessed 8,000 spent nuclear fuel rods, which would yield enough plutonium for up to six nuclear bombs. The CIA says Pyongyang may have already possessed one or two crude nuclear bombs.

Diplomatic efforts to open the second round of six-nation talks aimed at resolving the nuclear crisis have so far failed amid differences over the scope of negotiations.

The first round -- which brought together the United States, the two Koreas, China, Russia and Japan -- ended inconclusively in Beijing last August.

A second round had been tentatively pencilled in for December, but now looks unlikely to take place before February.

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