The UN atomic agency said Tuesday that some of its computers were infected by malicious software, in its second embarrassing IT slip-up over the past year.

The International Atomic Energy Agency, which holds highly sensitive information on its member states' nuclear facilities, said however that none of its data was compromised.

"During the past months, some computers operated by the IAEA have been infected by malware," spokesman Serge Gas said. "No data from the IAEA network has been affected."

The malware hit data on the USB drives of visitors to the IAEA in Vienna, although the devices themselves were not infected and did not spread the bug further, the IAEA believes.

Last November, the IAEA revealed that hackers had accessed one of its computer servers and posted the contact details of some of the watchdog's experts online.

"Protecting information is vital to the IAEA's work. The agency continuously endeavours to achieve the highest possible level of protection of information," Gas said.