A remote desert site in Syria that was bombed by Israeli planes in September 2007 was "very likely" a nuclear reactor, the UN atomic watchdog said Tuesday.

"Based on all the information available to the agency and its technical evaluation of that information, the agency assesses that it is very likely that the building destroyed at the Dair Alzour site was a nuclear reactor which should have been declared to the agency," the International Atomic Energy Agency said in a new restricted report, a copy of which was obtained by AFP.

The nine-page report was the toughest ever by the IAEA since it began investigating allegations of illicit nuclear work by Syria in 2008 and reflects the agency's growing frustration with Damascus, diplomats said.

Indeed, it is the first time that the IAEA has publicly stated its belief that Syria was building an undeclared reactor at Dair Alzour.

And diplomats suggest the report could now pave the way for Western powers at the upcoming board of governors meeting next month to push for Syria to be referred to the UN Security Council.

A senior international official familiar with the IAEA's investigation said the agency felt it had no option but to make such an assessment after Damascus has persistently refused to cooperate since the very beginning.

"We have given Syria ample opportunities to react, to engage with us. They didn't do that. I think we've exhausted all the possibilites. And so now we've made this assessment," the official said, speaking on condition of anonymity.

Even an unprecedented letter from IAEA chief Yukiya Amano to Syria's foreign minister late last year failed to "unblock the situation," the official said.

Damascus has indeed stonewalled the IAEA's investigation all along, granting inspectors access to the site only once in June 2008 and not allowing any follow-up visits to either Dair Alzour or other possible related sites.

It maintains that Dair Alzour was a non-nuclear military installation and that the IAEA therefore had no right to go there.

But suspicions only deepened when Syria cleared and hid all the debris from the site.

Furthemore, at their one and only visit to Dair Alzour, UN inspectors detected "significant" traces of man-made uranium there, as yet unexplained by Damascus.

In its new report, the IAEA said features of the destroyed building were "comparable to those of gas-cooled graphite-moderated reactors."

Indeed, photos of the building prior to the bombing showed a marked resemblance to North Korea's reactor at Yongbyong, which produced plutonium for Pyongyang's small stockpile of nuclear weapons.

The infrastructure at the site -- including its connections for cooling and treated water -- was configured in such a way as to support the operation of such a reactor and was "not consistent with Syria's claims regarding the purpose of the infrastructure," the report said.

The IAEA said the circumstances relating to the Dair Alzour site were unique "in that the building on the site has been destroyed, the debris from the site has been cleared, several years have now passed, and Syria has not provided the necessary cooperation required by the agency."

It was therefore forced to conclude that "after considering the initial allegations and Syria's responses thereto and considering all information available to the agency ... the destroyed building was very likely a nuclear reactor and should have been declared by Syria."