Siegfried Hecker, former head of the Los Alamos National Laboratory which developed the atom bomb, gave the Senate Foreign Relations committee a private account of his trip, congressional sources said.

Several congressional staffers declined to give details of his testimony.

Hecker 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.

North Korea has said it showed its "nuclear deterrent" to their US visitors and US newspapers later reported that the visitors appeared to have seen reprocessed plutonium, a key ingredient for making nuclear bombs.

Hecker was part of two non-government 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.

The group also included former State Department envoy to talks on North Korea Jack Pritchard. A second delegation consisted of congressional staffers Keith Luse and Frank Jannuzi, who also briefed the committee on Tuesday.

North Korea said it had shown a "nuclear deterrent" to US delegates while US newspapers reported that Pyongyang disclosed plutonium, an ingredient for a potential nuclear weapon, to the US delegations.

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

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