The International Atomic Energy Agency has acknowledged one of its computer servers has been hacked by an anti-Israeli group.
The agency says a previously unknown group calling itself Parastoo - which is Farsi for the bird swallow and an Iranian girl's name - posted details of more than 100 nuclear experts on its own website, calling for them to sign a petition for an IAEA investigation into Israel's Dimona reactor.
A statement dated 25 November said: "Israel owns a practical nuclear arsenal, tied to a growing military body."
The IAEA is investigating Iran's nuclear programme.
Israel is widely believed to have a nuclear arsenal, but neither confirms nor denies this under a policy of "strategic ambiguity".