ST PETERSBURG, Russia (Reuters) - Russia said on Wednesday that a military strike on Syria could have catastrophic effects if a missile hit a small reactor near Damascus that contains radioactive uranium.

The Foreign Ministry called on the U.N. nuclear agency to urgently assess the risk as the United States considers military action to punish Syria's government for an alleged gas attack.

"If a warhead, by design or by chance, were to hit the Miniature Neutron Source Reactor (MNSR) near Damascus, the consequences could be catastrophic," a ministry statement said.

It said nearby areas could be contaminated by highly enriched uranium and that it would be impossible to account for the nuclear material after such a strike, suggesting it could fall into the hands of people who might use it as a weapon.

Russia urged the U.N. International Atomic Energy Agency secretariat to "react swiftly" and present IAEA members "an analysis of the risks linked to possible American strikes on the MNSR and other facilities in Syria".

Moscow has been the most powerful ally of Syrian President Bashar al-Assad, shielding him from tougher U.N. resolutions and warning that a Western military attack on Syria would raise tensions and undermine efforts to end the country's civil war.

"The IAEA is aware of the statement but has not received a formal request from the Russian Federation," an IAEA spokesperson said. "We will consider the questions raised if we receive such a request."

The IAEA said in a report to member states last week that Syria had declared there was a "small amount of nuclear material" at the MNSR, a type of research reactor usually fuelled by highly enriched uranium.

Nuclear expert Mark Hibbs, of the Carnegie Endowment think-tank, said the MNSR was a very small reactor and there would not be a lot of nuclear material there.

But he said there could be "a serious local radiation hazard" if there was irradiated nuclear material in the reactor and it was dispersed by a weapon strike.

Olli Heinonen, a former IAEA chief inspector, said the core of such a reactor typically has 1 kg of highly-enriched uranium, much less than the 25 kg that would be sufficient to build an atomic bomb.

"Thus for nuclear explosive purposes it is of a limited value," he said in an e-mailed comment. Any radioactive contamination, he added, "would be a local problem".

In 2007, Israel bombed a desert site in Syria that U.S. intelligence reports said was a nascent, North Korean-designed reactor geared to producing plutonium for nuclear weapons. Syria said the site, at Deir al-Zor, was a conventional military facility.

(Additional reporting by Fredrik Dahl in Vienna; writing by Steve Gutterman; editing by Tom Pfeiffer and Ralph Boulton)