United Nations inspectors have commended Libya for working with them as they begin checking if it has really renounced its ambitions to build an atomic bomb.
"Libya has shown a good deal of cooperation, a good deal of openness," said Mohamed ElBaradei, the head of the International Atomic Energy Agency (IAEA).
He headed a team of inspectors invited to the North African country to see how far it had got in developing a bomb and to make sure it goes no further.
In a shock move following months of secret talks with US and British officials, Libya said this month it was giving up efforts to obtain nuclear, chemical or biological weapons.
Mr ElBaradei said Libya, led by Muammar Gadafy, had agreed to sign the additional protocol to the international nuclear Non-Proliferation Treaty, allowing more intrusive snap inspections.
"Libya committed today to act as if the protocol was in force," he told reporters before leaving Tripoli for Vienna where the IAEA is based. Other inspectors will stay on.
In a first full day of inspections on Sunday, inspectors visited four nuclear sites near the capital Tripoli that the UN body had never seen before.
Libya, long on the U.S. list of sponsors of terrorism and treated as a pariah by the West, displayed dismantled and boxed uranium enrichment centrifuges - machines which can purify the radioactive material for use in weapons or as nuclear fuel.
"What we have seen is a programme at a very initial stage," Mr ElBaradei said of the enrichment programme. "I am happy that we came in at that stage."