The United States said Wednesday it views with "tremendous concern" a UN report that a remote desert site in Syria that was bombed by Israeli planes in 2007 was probably a nuclear reactor.

The International Atomic Energy Agency (IAEA) said in a report 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 attempt by Syria to construct a clandestine nuclear reactor site is obviously a matter of concern, and we fully expect that the IAEA board will address this issue when it meets," State Department spokesman Mark Toner said.

"The IAEA board will meet and then decide whether to advance it to the UN Security Council," Toner told reporters without actually saying that Washington would push for it to go to the council.

"We view this as a matter of tremendous concern and will make our views clear at the board meeting," he said.

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.