Washington alleges that Syria was constructing the reactor at a remote desert site called Dair Alzour with North Korea's help until the site was bombed by Israeli planes in September 2007.

White House spokesman Jay Carney said the decision by the International Atomic Energy Agency's Board of Governors marked a "significant action by the international community to uphold the nonproliferation rules of the road."

"As the IAEA reports, with assistance from North Korea, Syria attempted to build a secret nuclear reactor capable of producing large amounts of nuclear weapons-usable plutonium, but with no apparent legitimate civilian purpose."

"Syria has stonewalled and obstructed the efforts of the IAEA to investigate the nuclear reactor for years," Carney said.

At a closed-door meeting of the 35-member board of governors of the IAEA in Vienna, 17 countries voted in favour of a corresponding resolution by the United States and six against, diplomats attending the meeting said.

Azerbaijan, China, Ecuador, Pakistan, Russia and Venezuela all voted "no".

The IAEA began investigating the allegations in June 2008, but Syria has refused to cooperate and, with the exception of a one-off visit, has not allowed UN inspectors to Dair Alzour or related sites to verify the US claims.

Washington tabled the resolution at a time when it has been critical of Syria's crackdown on anti-government protests, which human rights groups say has killed more than 1,100 people.

Washington and its Western allies have asked the 35-member board of governors of the International Atomic Energy Agency to find Syria in "non-compliance" with its international obligations and report it to the UN Security Council in New York.

But in statements to fellow board members ahead of a vote on the matter later Thursday, both Moscow and Beijing said they saw no reason for such action.

"The UN Security Council is responsible for holding international peace and security and the site at Dair Alzour no longer exists and therefore poses no threat to international peace and security," the Russian statement said.

"We cannot therefore agree with the resolution and that is why, if it is put to a vote, we will vote against it," it said.

China followed simliar arguments, saying it saw "no reason to adopt the resolution or refer Syria to the Security Council."

Beijing did not specify, however, whether it planned to vote against the resolution or abstain from the vote.

Washington alleges that Syria was building an undeclared nuclear reactor at a remote desert site called Dair Alzour with North Korea's help until the site was bombed by Israeli planes in September 2007.

The IAEA began investigating the allegations in June 2008, but Syria has refused to cooperate all along and, with the exception of a one-off visit, has not allowed UN inspectors to Dair Alzour or related sites to verify the US claims.

Frustrated by Syria's three years of stonewalling, IAEA chief Yukiya Amano took the unprecedented step recently of stating his conviction that the site was "very likely" a covert nuclear reactor, as alleged.

Washington seized on Amano's assessment and tabled a resolution to censure Damascus at the regular June meeting of the IAEA board of governors this week.

"Syria's apparent attempt at constructing a covert, undeclared plutonium production reactor, a reactor with no credible peaceful purpose, represents one of the most serious safeguards violations possible," US ambabassador Glyn Davies told the IAEA board of governors.

"Syria's nuclear intentions at Dair Alzour are clear; the reactor there was built for the express purpose of producing plutonium for possible use in nuclear weapons," Davies said, arguing a resolution was "the only responsible course of action."

Western diplomats insist they have sufficient support on the 35-member board for the resolution to be carried by a simple majority. The last time a state was referred to the UN Security Council in New York was Iran in February 2006.

But with both Russia and China veto-wielding members of the Security Council, their objection could lessen the impact of such a move.

"The draft resolution is untimely and not objective," the Russian statement said.

It argued that a country could only be found in non-compliance if there were an exchange of nuclear material, which was not the case here.

Furthermore, it was not for IAEA chief Amano to make an assessment on the likely existence of a nuclear reactor.

"The agency shall not make conclusions on the basis of suppositions or speculations, especially in the situation where there is a lack of specific information," the Russian side said.

The resolution is co-sponsored by Australia, Belgium, Britain, Canada, the Czech Republic, Denmark, France, Germany, Italy, the Netherlands, Portugal and South Korea.

Syria had been reeling from anti-government protests since mid-March, with human rights groups saying more than 1,100 people have been killed throughout the country.