Western and Israeli security experts suspect Syria may have tonnes of unenriched uranium in storage and that any such stockpile could potentially be of interest to its ally Iran for use in Tehran's own disputed nuclear program.
They say natural uranium could have been acquired by the Arab state years ago to fuel a suspected nuclear reactor under construction that was bombed by Israel in 2007.
U.S. intelligence reports at the time said the site in Syria's desert Deir al-Zor region was a nascent, North Korean-designed reactor designed to produce plutonium for atomic arms.
Syria, ravaged by a war the United Nations says has killed 60,000 people, has denied accusations of a clandestine nuclear programme. Its envoy in Vienna, where the U.N. nuclear watchdog is based, was not available for comment on Friday.
"Someplace there has got to be an inventory of fuel for the reactor. It doesn't make sense to have a nuclear installation, a nuclear reactor, without any fuel," proliferation expert Mark Hibbs of the Carnegie Endowment think tank said.
But, he added, "to my knowledge there hasn't been any substantiated accounts identifying where that material may be located." It would likely have come from North Korea, he said.
Even if Syria did have such a stockpile, it would not be usable for nuclear weapons in its present form, a fact that makes it less of a pressing concern for the West than fears that government forces may use chemical arms against their foes.
The Financial Times newspaper said this week Syria may hold up to 50 tonnes of unenriched, or natural, uranium - material which can fuel atomic power plants and also provide the explosive core of nuclear bombs, but only if refined to a high degree.
Some government officials have raised concerns that Iran might try to seize it, the FT said, without identifying them.
Though such a quantity in theory could yield material for several atom bombs, it would first have to be enriched much further, from 0.7 percent of the fissile isotope in natural uranium to 90 percent, in a technically complicated process.
Iran, which denies Western accusations of atomic bomb ambitions, has said its mines can supply the raw uranium needed for its nuclear programme and that it has no shortage problems.
The U.N.'s International Atomic Energy Agency (IAEA), which for several years has been seeking access to the destroyed Deir al-Zor site as well as three other locations that may be linked to it, declined to comment on the FT report.
A recently retired Israeli security official said he believed Syria was keeping uranium at a site near Damascus, one of the places the IAEA wants to inspect, but he did not say what he based this on.
The former Israeli official said rebels fighting Syrian President Bashar al-Assad, who now control a crescent of suburbs on the outskirts of the capital, may get hold of the stockpile and make its existence public.
"Then it would put paid to the Syrians' claims that they never had a reactor in the first place," he said.
Another possibility was that Syria, "knowing the material is no longer secured, could ship it out to Iran, which is certainly in need of more uranium for its own nuclear plans," the former Israeli official, who declined to be named, added.
But a veteran Israeli intelligence analyst who now works as a government adviser said the figure of 50 tonnes of uranium cited by the Financial Times was "not at all familiar to me".
A Western diplomat said there had been speculation about possible uranium - perhaps in the form of natural uranium metal to fuel a reactor - in Syria because of the destroyed Deir al-Zor site but that he knew of no specific details.
"It is plausible. But as far as I know no one has ever had any idea where the material is," he said, adding it would not be easy to ship large quantities to Iran without detection.
Syria says Deir al-Zor was a conventional military facility but the IAEA concluded in May 2011 that it was "very likely" to have been a reactor that should have been declared to its anti-proliferation inspectors.
If there is a stockpile of uranium in Syria, it would be of use for Iran as it faces a potential shortage, said Mark Fitzpatrick, a proliferation expert at the International Institute for Strategic Studies (IISS) think-tank.
"Syria has been getting quite a bit of help from Iran. This would have been one means of repaying them," he said. "There is evidence that Iran is looking around the world for uranium."
Israel, which is widely believed to have the Middle East's only nuclear arsenal, and Western powers accuse Iran of seeking to develop a capability to make atomic bombs.
The Islamic state says its programme to refine uranium is solely intended for peaceful energy and medical purposes.
Some Western analysts have said Iran may be close to exhausting its supply of raw uranium, known as "yellow cake", although IAEA reports suggest it still has plenty of natural uranium gas to use for its enrichment work.
"If there is an undeclared inventory of 50 tonnes of uranium then, if I were Assad, I would want to spirit it out of there and the most likely place would be Iran," Hibbs said.