VIENNA (AP) — The International Atomic Energy Agency acknowledged Tuesday that one of its servers had been hacked after a previously unknown group critical of Israel's undeclared nuclear weapons program posted contact details for more than 100 experts working for the U.N. nuclear watchdog.

A group called "Parastoo" — Farsi for the swallow bird and a common Iranian girl's name — claimed responsibility for posting the names on its website two days ago.

Israel is commonly acknowledged to possess nuclear weapons but has neither confirmed or denied its status. It says Iran is secretly working to make nuclear arms — something Tehran denies — and describes the Islamic republic as the greatest threat to the Mideast. But Iran and Arab countries say the Jewish state's nuclear capacities pose the greatest menace.

Chastising Israel for its "nuclear arsenal," the hackers urged the experts whose names they published to sign a petition demanding an "open investigation" into Israel's nuclear program.

IAEA spokeswoman Gill Tudor said the agency "deeply regrets this publication of information stolen from an old server." She said the server had been shut down some time ago and agency experts had been working to eliminate any "possible vulnerability" in it even before it was hacked.

The IAEA was doing "everything possible to help ensure that no further information is vulnerable," she said in an email.