International Atomic Energy Agency (IAEA) chief Muhammad al-Baradei told reporters his inspectors had been shown equipment for uranium enrichment which could have been used to purify the material for use in a weapon.
Libya said it bought the equipment on the black market, he said.
"Libya has shown a good deal of cooperation, a good deal of openness," al-Baradei said before leaving Libya for his base in Vienna after a short visit to the North African country.
Libya said this month it was abandoning plans to build an atomic bomb and other banned weapons and invited inspectors in. The UN watchdog head said Libya had agreed to sign the additional protocol to the nuclear Non-Proliferation Treaty, allowing more intrusive snap inspections than called for under the main treaty.
"Libya committed today to act as if the protocol was (already) in force," he said after a first full day of inspections.
"What we have seen is a programme at a very initial stage," al-Baradei said of the Libyan uranium enrichment programme, which would have been the heart of any atom bomb ambitions Tripoli may have had. "I am happy that we came in at that stage."
Asked about the uranium enrichment centrifuges which the UN inspectors saw dismantled and boxed, he said: "They are familiar. We can identify the designs, and they are quite sophisticated."
Al-Baradei said UN inspectors had already interviewed several Libyan scientists.