The United States said on Monday it disagreed with a Russian request for the UN nuclear agency to analyze the possible risks involved if a Syrian reactor were to be hit during military strikes.
Russia said last week any US-led military action against Syria's government could have catastrophic effects if a research reactor near Damascus that contains radioactive uranium was struck "by design or by chance".
Russia's Foreign Ministry called on the International Atomic Energy Agency (IAEA) to urgently assess the risk as the United States considers a punitive military strike in Syria over an alleged poison gas attack in its civil war.
In a statement to an IAEA board meeting on Monday, US Ambassador Joseph Macmanus said: "It is our view that requests for comprehensive risk analyses of hypothetical scenarios are beyond the IAEA's statutory authority."
The IAEA "will have to review such a request in light of legal authorities, mandate and resources and must determine whether there is a scientific basis for conducting a highly speculative investigation of this kind," Macmanus said, according to a copy of his speech.
Moscow is the Syrian government's biggest ally and arms supplier. The United States supports rebels fighting to overthrow Syrian President Bashar al-Assad.
IAEA Director General Yukiya Amano earlier told the board the UN agency was considering the Russian request.
Nuclear experts say the so-called Miniature Neutron Source Reactor (MNSR), a type of research reactor that is usually fuelled by highly enriched uranium, is small and that any radioactive fallout may pose a local hazard.
The amount that such a reactor usually holds, about 1 kg of highly enriched uranium, is less than the 25 kg that would be sufficient to build a bomb, they say.