The UN's nuclear watchdog, the IAEA, has said questions remain about Libya's nuclear weapons programme, despite its promise last December to scrap it.
Libya has opened up its research facilities to international inspection
In a confidential report, the IAEA said it was happy with Libya's co-operation but that investigations must continue.
It said some of Libya's nuclear equipment had been contaminated with highly enriched uranium, which can be used to make nuclear weapons.
But it said there was no evidence Libya had begun building a nuclear warhead.
It also said it was surprising Libya had not acted on the weapons design information it possessed, given the substantial effort it was devoting to uranium enrichment.
The leaked report linked Libya to an international black market in nuclear equipment run by the Pakistani scientist AQ Khan.
The IAEA said it found the uranium on centrifuges which Libya bought on the black market network.
"It is clear... that the existence of this procurement
'network' was of decisive importance in Libya's clandestine nuclear weapons programme," the report said, according to Reuters news agency.
The finding could support Iran's insistence that bomb-grade uranium found on its centrifuges last year came from the black market.
The report also noted that a container of components of an advanced kind of centrifuge arrived in Libya in March this year, several months after Tripoli announced it was giving up nuclear weapons.
Libya notified the agency of the arrival of the container and it was shipped out of the country.