A U.N. investigation has formally concluded that Syria “very likely” was building a secret nuclear reactor in 2007 when the partially completed project was destroyed by Israeli warplanes, according to a report Tuesday that put new pressure on the embattled government of President Bashar al-Assad.

The report by the International Atomic Energy Agency was the first by the watchdog group to officially link Syria to a clandestine nuclear program, a move that opens the door to punitive measures, including a possible referral to the U.N. Security Council for sanctions.

The United States and its allies have maintained for years that the construction site attacked by Israel on Sept. 6, 2007, was a nuclear reactor intended for making fuel for nuclear bombs, a claim that Syria has vigorously denied. Tuesday’s IAEA report cites extensive physical and photographic evidence showing that Syria’s Dair Alzour project had all the hallmarks of a 25-megawatt nuclear reactor.

U.N. officials determined that the partially completed reactor on the banks of the Euphrates River was not configured to produce electricity, suggesting that it was almost certainly part of a clandestine nuclear weapons effort.

“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,” IAEA investigators concluded in the confidential report prepared for the U.N. watchdog’s board of governors.

The U.N. report criticized Syria for stonewalling its investigation for much of the past three years by blocking access to key sites and presenting false or misleading information about the destroyed facility.

While Syria has claimed that the complex was for missile storage and maintenance, the IAEA cited satellite photos, taken before and after the bombing, that clearly showed a large containment vessel and other features specific to nuclear reactors. The report accused Syria of concealing the building’s intended use by burying power lines and disguising design features that would have been easy to spot from space.

The United States and its European allies have been calling for months on the U.N. agency to resolve the long-standing controversy over the nature of the Syrian facility.

U.S. officials are expected to seize on the IAEA’s new report to push for a vote by the watchdog group’s 35-nation governing body to find Syria in violation of the U.N. nuclear Non-Proliferation Treaty, Western diplomats said Tuesday.

The United States and its European allies have no immediate plans to push for a Security Council response, saying they fear it might complicate ongoing efforts to pass a Security Council condemnation of Syria for its bloody crackdown on protesters.

France and Britain are planning to press for a Security Council vote on a mild resolution that would condemn Syria for repressing anti-government demonstrations. It would demand that the government show restraint and grant the United Nations access to the country to provide humanitarian assistance, according to U.N. diplomats.

French President Nicolas Sarkozy and British Prime Minister David Cameron will make one last effort to persuade Russian President Dmitry Medvedev not to veto the resolution, according to council diplomats. But they said they intend to press for a vote even if Russia threatens to veto it.

Staff researcher Julie Tate in Washington and staff writer Colum Lynch at the United Nations contributed to this report.