The head of the UN atomic agency said Thursday that he was still in the dark over a hacking incident but that he was "confident" no sensitive information related to its inspections work was stolen.

"We are continuing to analyse this event and I don't claim that I have all the knowledge of what happened. We are continuing to analyse the case," International Atomic Energy Agency chief Yukiya Amano said.

"We don't know everything but we are confident that safeguards information have not been stolen. But we don't know anything yet, we are analysing it," he told a news conference.

"Safeguards" relates to the Vienna-based body's activities in ensuring that nuclear weapons technology and knowhow is not spread.

Amano revealed that the hacking took place several months ago, calling it "deeply regrettable".

Previously the agency had not confirmed when the incident happened, saying only that the server was old and had since been shut down and that the hackers had posted "some contact details" on the Internet.

A group calling itself Parastoo, which means "swallow" in Farsi, this week posted email addresses of a list of experts working with the IAEA, with comments critical of Israel's nuclear activities.

The group said in an online statement it had "grabbed a total backup" and that it was "reassuring (the) IAEA that their critical information is safe with us".

Israel, widely assumed to have nuclear weapons but which has never confirmed or denied it, is not a signatory to the nuclear Non-Proliferation Treaty (NPT) and -- unlike Iran -- is not obliged to grant access to IAEA inspectors.