Syria supports Russia's proposal for the UN atomic agency to assess the risks posed by any US-led airstrikes hitting a small research reactor outside Damascus, Syria's ambassador to the IAEA said Tuesday.

"We join strongly the Russian Federation demand requesting analysis from the IAEA on the risk of such an attack on the facilities," Bassam al-Sabbagh told reporters.

"Also we assure the (IAEA) director general (Yukiya) Amano that the IAEA has the mandate and the responsibility" to conduct the analysis, he said at the International Atomic Energy Agency headquarters in Vienna.

The United States opposes the IAEA carrying out what its ambassador on Monday called a "highly speculative investigation", saying this lay outside the watchdog's responsibilities.

Many diplomats in Vienna see Russia's move as an attempt to throw an obstacle in the way of Washington carrying out strikes on Syria in response to an alleged chemical weapons attack on August 21.

Experts also say that Moscow's assertion that the effects of airstrikes hitting the Miniature Neutron Source Reactor (MNSR) could be "catastrophic" is exaggerated.

Robert Kelley from the Stockholm International Peace Research Institute (SIPRI) said the core of the research reactor, in desert some 15 kilometres (nine miles) outside the Damascus city limits, "is about the size of a can of paint."

"If the building were somehow attacked, the reactor is at the bottom of a deep well. Unless the bomb went exactly down this narrow well, the result would be to collapse the building and earth on the reactor where it would sit doing absolutely nothing until someone could dig it up again," Kelley told AFP.

"There would be zero effects on the local area. There will be no casualties and no contamination... Bombing the reactor probably makes it safer."

He added however that it is also the site of a large food irradiation facility using powerful gamma sources -- probably in shielded vaults above ground -- to sterilise food, largely dates.

If these materials were scattered by a bombing, this would represent a local hazard and necessitate a "nasty clean-up" of the immediate vicinity of hundreds of metres (yards), he said.

Radioactive sources such as this are widespread, not just in Syria but worldwide -- including in countries bombed in recent years -- in hospitals, research laboratories and in industry.

The IAEA has said it is considering the Russian request. Amano said Monday that the reactor contained around one kilo (two pounds) of highly-enriched uranium, calling this "not a big amount".

According to the IAEA, the reactor's fuel is weapons-grade: highly enriched uranium (HEU) enriched to 90-percent fissile purities, supplied by China.

But triggering a nuclear explosion using this uranium is a highly complex procedure and would not be triggered simply by being hit with conventional explosives.

The material could in theory be stolen in the chaos of Syria's civil war, but using it in a nuclear bomb is widely thought to be beyond the technical capabilities of any extremist group.

It could however be used in a so-called dirty bomb -- a conventional explosive device dispersing material -- which would not necessarily be particularly lethal but which might create panic.

"HEU is not very radioactive and when scattered, it would be hard to even find it, let alone measure any health effects," Kelley said.