Tarek Mitri said the uranium concentrate was stored at a former military installation near Sabha under the control of a Libyan army battalion.
With the backing of the UN Support Mission in Libya, which Mitri heads, "an inspection team from the IAEA will visit this month to verify existing stockpiles and conditions of storage," he said.
Former Libyan leader Moamer Kadhafi in 2003 renounced his efforts to acquire weapons of mass destruction, including nuclear weapons. But a large stockpile of yellowcake was found intact after the uprising that toppled his regime in 2011.
The UN nuclear watchdog however said there was little risk of theft because of the size and weight of the barrels.
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Yellowcake, a powder consisting of around 80 percent uranium oxide, is used in the preparation of nuclear fuel for reactors. It also can be enriched for use in nuclear weapons.
Mitri also said a mission from the Organisation for the Prohibition of Chemical Weapons will go to Libya this month to observe and verify the destruction of Libyan chemical weapons.
Concerning the Kadhafi regime's enormous arsenal of rocket launchers, Mitri said he had received preliminary reports but had asked the Libyan authorities to provide greater documentation.