* Hacking incident happened months ago, U.N. nuclear chief says
* Sensitive data not stolen, server closed down - Amano
* IAEA takes steps to prevent any new hacking attack
By Fredrik Dahl
VIENNA, Nov 29 (Reuters) - The head of the U.N. nuclear agency said on Thursday a hacking incident involving one of its servers was "deeply regrettable" but suggested that no sensitive information related to Iran's atomic activities had been stolen.
Yukiya Amano said the hacking - first reported by the International Atomic Energy Agency (IAEA) on Tuesday - had happened several months ago and that the server concerned had been closed down.
The hackers - a group using an Iranian-sounding name - on Sunday posted scores of email addresses of experts who have been working with the U.N. agency on a website, and urged the IAEA to investigate Israel's nuclear activity.
The U.N. agency did not say who it believed might have been behind the hacking. There has been an increase in suspected Iranian cyber attacks this year, coinciding with a deepening standoff with the West over Tehran's nuclear programme.
The most worrying, experts say, were those on Saudi oil firm Aramco - effectively destroying tens of thousands of computers - and Qatari gas export facilities.
Iranian officials have tended to deny involvement. But they say they have continued to come under cyber assault themselves, with systems at Iran's own oil facilities, communications and infrastructure firms suffering problems last month.
Amano told a news conference about the attack involving the IAEA. "We don't know everything (about it) but we are confident that safeguards information has not been stolen ... We believe confidential safeguards information has not been compromised."
"Safeguards" means activities conducted by IAEA inspectors in examining member states' nuclear activities, including Iran's, to make sure that no atomic material is diverted for military purposes. Such information is seen as top secret.
The Vienna-based agency is investigating suspicions that Iran has carried out research relevant for the development of nuclear weapons, a charge the Islamic Republic denies.
Amano said the IAEA was taking measures to prevent the re-occurrence of any hacking, adding that it was continuing to analyse what had happened.
A diplomat accredited to the IAEA said: "The breach of security, although as I understand it does not involve confidential tech information, is serious."
The hacker statement - posted in the name of Parastoo, which in Farsi means swallow, a species of bird, or can be a girl's name - called for the people to whom the email addresses belonged to sign a petition for an "open" IAEA investigation into Israel's Dimona reactor.
Dated Nov. 25 and headlined "Parastoo Hacks IAEA", it said: "Israel owns a practical nuclear arsenal, tied to a growing military body."
Israel is widely believed to have the Middle East's only atomic arsenal but neither confirms nor denies this under a "strategic ambiguity" policy to deter Arab and Iranian foes.
The country is outside the Non-Proliferation Treaty that would require it to forswear nuclear weapons and open up its reactor in the southern desert town of Dimona.
Israel and the United States accuse Iran of seeking to develop a nuclear weapons capability. Tehran denies this.
Iran and Arab states say Israel's assumed atomic arsenal threatens peace and security in the Middle East.