International Atomic Energy Agency Director-General Muhammad al-Baradei, flying to Tripoli from IAEA headquarters in Vienna, said on Saturday there were no signs Libya had enriched uranium - a step that, were it taken, could be the first move to a bomb.
"From the look of it, they were not close to a weapon, but we need to go and see it and discuss the details with them," he said in an interview on the way to Tripoli.
"The important thing for me is to get a comprehensive understanding of the programme - the origin, its history, its extent, and then agree with the Libyan authorities on a plan of action to eliminate whatever needs to be eliminated that is not linked to peaceful activities."
Libyan leader Muammar Qadhafi's oil-rich state, long on the US list of "sponsors of terrorism", now wants trading benefits, including an end to US sanctions for his promise to abandon weapons of mass destruction.
Libya's moves to scrap its illicit weapons programmes mark an about-face for the mercurial Gaddafi, who seized power 34 years ago in the desert nation of 5.5 million.
Al-Baradei could begin talks with senior Libyan officials as early as Saturday afternoon.
Vienna-based diplomats who watch the IAEA said they believed al-Baradei would meet Qadhafi, who has pledged to let UN experts assess and dismantle banned weapons projects.
In a Vienna meeting this month, a senior Libyan official told al-Baradei that Tripoli had also embarked on a uranium enrichment programme, though it was only at the planning stage.
The official also said Tripoli, which has signed the 1968 nuclear Non-Proliferation Treaty (NPT), would soon sign an NPT protocol permitting more intrusive, short-notice checks.
"From the look of it, they were not close to a weapon, but we need to go and see it and discuss the details with them"
Several Western diplomats told Reuters they were aware of suspicions Libya was preparing a nuclear arms programme.
But they said US and British assertions that Tripoli was close to developing a nuclear weapon may have been exaggerated.
For much of Qadhafi's rule, Libya has been under US or UN sanctions, accused of sponsoring or carrying out terrorist acts ranging from bombing airliners to training foreign guerrillas.
UN sanctions were lifted this year after Libya agreed to pay compensation for the 1988 Lockerbie airliner bombing, which killed 270 people. But Washington kept its embargo in place.