"We have no reason to believe that President Pervez Musharraf or the top echelons of the Pakistani government were in any way involved with Khan," US Undersecretary of State John Bolton told reporters in Lisbon where he is taking part in a two-day security conference.

"There may well be officials in the Pakistani government, military people, scientists, who were part of his network," he added.

"But I distinguish between that and sanctioned approval, complicity, by the top levels of the Pakistani government as to which we have no evidence."

Khan, a national hero in Pakistan since he helped test the Islamic world's first nuclear bomb in 1998, confessed on television last month to passing on nuclear secrets after a two-month probe prompted by information from the International Atomic Energy Agency.

Musharraf has denied allegations that former army chiefs and governments authorised the nuclear sales, insisted that no proliferation had taken place since he seized power in a 1999 coup and has vowed that it would never happen again.

But he has refused to allow an inquiry by international investigators, who want to probe further to determine whether the nuclear know-how was sold to other countries.

Bolton said Washington could not rule out the possibility that Khan and his network had made out nuclear sales to other countries or that the trafficking was still continuing.

"We do not know the extent to which other parts of the network may have survived, we are still investigating," he said.

"There are reports that Khan was engaged with other countries and I wouldn't say these are reports that we feel we can confirm but it is obviously critical to try and understand the extent of his network's activities."

Bolton praised Musharraf, however, for taking what he described as "important steps" to disrupt the network and make sure that the nuclear proliferation stops.

The Pakistani president pardoned Khan after he made the televised confession and exonerated the military of any role.

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