The United States and Russia, already at odds over Syria - clashed further on Monday over whether the UN nuclear agency should analyse the possible risks involved if a reactor near Damascus were to be hit during US-led military strikes.
Russia said last week any military action against Syria's government could have catastrophic effects if a research reactor near the Syrian capital 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 issue with the United States preparing for a punitive air strike in Syria over an alleged poison gas attack in its civil war.
But in a statement to an IAEA board of governors meeting on Monday, where Moscow reiterated its demand, US Ambassador Joseph Macmanus made Washington's objections clear.
"It is our view that requests for comprehensive risk analyses of hypothetical scenarios are beyond the IAEA's statutory authority," he said, according to a copy of his speech in the closed-door session.
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.
Moscow is the Syrian government's most powerful ally and main arms supplier and has blocked UN Security Council action on Syria sought by the United States and Western allies, who back rebels fighting to oust Syrian President Bashar al-Assad.
IAEA Director General Yukiya Amano earlier told the 35-nation board that the UN agency was considering the Russian request, describing it as a "complicated issue" with technical, political as well as legal aspects.
Russia had asked for a quick response but "I hope people understand it takes time", Amano told a news conference later.
"The views are divided so far," he said about statements made on the issue by Russia, the United States and Cuba during the board meeting, which is due to end on Friday and will also debate Iran and other topical nuclear matters.
He said the Syrian reactor - which IAEA inspectors have visited in the past - held about 1 kg (2.2 pounds) of highly-enriched uranium (HEU), saying this was not a "big amount".
Nuclear experts say the so-called Miniature Neutron Source Reactor (MNSR), a type of research reactor that is usually fuelled by HEU, is small but that any radioactive fallout might pose a local hazard.
The kilogram of HEU that such a reactor usually holds is only a small fraction of the 25 kg (55 pounds) that would be sufficient to assemble a single nuclear bomb, they say.
In a letter to other member states seen by Reuters, Russia said it had asked the IAEA to "react without further delay to the current situation and to provide member states with full analysis of risks associated with possible American strikes on MNSR and other sites in Syria".
But Macmanus, the US envoy, said: "The IAEA has never before conducted this type of analysis. It would exceed the IAEA's mandate, (and) has far-reaching implications that exceed IAEA capabilities and authorities."-Reuters