The U.N. nuclear watchdog Tuesday acknowledged that one of its servers had been hacked, after the hackers posted the stolen data online earlier this week.

The International Atomic Energy Agency (IAEA) said a previously unknown anti-Israeli group called Parastoo — meaning the swallow bird in Farsi and a common name for girls in Iran — had posted the contact details of more than 100 nuclear experts on the group's website three days ago.

A statement accompanying the contact details asked the experts, whose names were published, to sign a petition demanding an "open investigation" into Israel's undeclared nuclear weapons program. The U.N. agency is currently investigating Iran’s alleged nuclear program.

Israel is widely acknowledged to possess nuclear weapons but has neither confirmed or denied its status and is not a party to the Nuclear Non-Proliferation Treaty (NPT). The U.S. government also officially does not acknowledge the existence of the nuclear weapons in Israel.

Israel’s official position, as reiterated by Aaron Sagui, spokesman for the Israeli Embassy in Washington, is that “Israel will not be the first country to introduce nuclear weapons into the Middle East. Israel supports a Middle East free of all weapons of mass destruction following the attainment of peace.” The “introduce” language is purposefully vague, but experts say it means that Israel will not openly test a weapon or declare publicly that it has one, according to the Washington Post.

Since Israel is not a signatory of the NPT, it is not obliged to submit its nuclear facility in Dimona to the IAEA inspections. Iran, on the other hand, has signed the treaty thus agreeing to periodic inspections.

Israel considers the nuclear Iran an existential threat while Iran and other Arab countries say the Jewish state's nuclear capabilities put the region at risk.

IAEA spokeswoman Gill Tudor said the agency "deeply regrets this publication of information stolen from an old server.”

"The IAEA's technical and security teams are continuing to analyze the situation and do everything possible to help ensure that no further information is vulnerable," she added.

A Western diplomat told Reuters that the stolen data was not believed to include information relating to the confidential work carried out by the IAEA.