The US and Russia clashed at the UN atomic agency Monday over Moscow's request for an IAEA probe into the risks posed by US airstrikes hitting a small Syrian reactor.
The head of the International Atomic Energy Agency, Yukiya Amano, said meanwhile that there was "not a big amount" of radioactive material at the research reactor in the suburbs of Damascus.
The US ambassador to the IAEA told a closed-door meeting of the watchdog in Vienna that carrying out Russia's request was not the UN agency's job.
Joseph Macmanus said that "requests for comprehensive risk analyses of hypothetical scenarios are beyond the IAEA's statutory authority," according to a text of his remarks obtained by AFP.
"The IAEA has never before conducted this type of analysis, it would exceed IAEA's mandate, has far-reaching implications that exceed IAEA capabilities and authorities," he said.
While not mentioning Syria by name, Macmanus said the agency "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."
Permanent UN Security Council member Russia opposes US-led airstrikes in response to an alleged use of chemical weapons by the regime of President Bashar al-Assad on August 21.
Last week Russia warned that US strikes could have "catastrophic" consequences if the Miniature Neutron Source Reactor (MNSR) were hit either "deliberately or inadvertently".
In a letter to IAEA member states seen by AFP, Russia said there was a "probability of contamination of (the) surrounding area ... as well as practical inability to ensure further accounting, control and security of the nuclear material on this site".
Amano said Monday that the IAEA was considering Russia's request for a risk analysis.
He said that the agency had to examine a number of legal, technical and political aspects before responding, telling reporters it was "a complicated issue".
He said however that the reactor contained around one kilo (two pounds) of highly enriched uranium, saying that this was "not a big amount". He declined to comment however on the possible implications of a military strike.
He added that initial reactions from IAEA member states to Russia's request, reiterated by Moscow's ambassador in Monday's meeting, were "divided".
One diplomat told AFP on condition of anonymity that Russia's request to the IAEA was a "diplomatic manoeuvre" and that the risks were "minimal".
"Russia is pulling out all the stops to deter a US attack on Syria and apparently it feels this is one of them," Mark Hibbs, analyst at the Carnegie Endowment for International Peace, told AFP.