The UN nuclear agency on Thursday said for the first time that a target destroyed by Israeli warplanes in the Syrian desert five years ago was a covertly built nuclear reactor, countering assertions by Syria that it had no atomic secrets to hide.

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Previous reports by the International Atomic Energy Agency have suggested that the structure hit could have been a nuclear reactor. Thursday's comments by IAEA chief Yukiya Amano were the first time the agency has said so unequivocally

He made them during a news conference meant to focus on the Fukushima nuclear disaster after a visit to the Paris-based Organization for Economic Cooperation and Development to discuss clean-up efforts at Japan's tsunami-ravaged nuclear plant.

"The facility that was ... destroyed by Israel was a nuclear reactor under construction," he asked in response to a question from The Associated Press, repeating to the AP afterward: "It was a reactor under construction."

Israel has never publicly commented on the strike or even acknowledged carrying it out. The US has shared intelligence with the agency that identifies the structure as a nearly completed nuclear reactor that, if finished, would have been able to produce plutonium for the fissile core of nuclear warheads.

Syria denies allegations of any covert nuclear activity or interest in developing nuclear arms. Its refusal to allow IAEA inspectors new access to the bombed Al Kibar desert site past a visit four years ago has heightened suspicions that it had something to hide, along with its decision to level the destroyed structure and later build over it.

Drawing on the 2008 visit by its inspectors, the IAEA determined that the destroyed building's size and structure fit specifications that a reactor would have had. The site also contained graphite and natural uranium particles that could be linked to nuclear activities.

The IAEA is also trying to probe several other sites for possible undeclared nuclear activities linked to the bombed target but Damascus has been uncooperative on most counts, saying that most of the sites are restricted because of their military nature.