With Syrian President Bashar Assad's brutal regime looking increasingly shakier after 18 months of civil war, there's growing unease about the security of Syria's undeclared chemical weapons, which is reputed to be the fourth largest stockpile in the world.

Regional powers like Turkey and Jordan, as well as Western governments, say they fear Assad's minority Alawite regime might use chemical weapons against rebel forces, or neighboring states supporting them, if it looks like Assad is going to fall.

There are also deep concerns that some of the weapon depots could fall into the hands of extremist groups among the disparate opposition forces such as al-Qaida.

Israel and some Western security chiefs have suggested Assad or hard-liners in his inner circle might give chemical weapons to his ally, the Iranian-backed Hezbollah in Lebanon.

Those fears were heightened Monday when the German newsmagazine Der Spiegel reported the Syrian army is believed to have tested the firing systems on some chemical weapons system at the end of August.

The tests, which haven't been independently verified, reportedly took place at the regime's largest chemical arms facility at Safira, east of the war-torn northern city of Aleppo, under the supervision of Iranian military specialists.

The commander of Iran's Islamic Revolutionary Guards Corps, Maj. Gen. Mohammed Ali Jafari, declared Sunday that personnel from its elite Al-Quds Force, the IRGC's covert arm which operates outside Iran, were in Syria aiding Assad's forces.

Jafari maintained the support didn't include military operations but said it included "transferred experience."

The IRGC is believed to have a chemical weapons branch. It used chemical arms against Iraq in a 1980-88 war.

Regional and Western intelligence services say they have little doubt that Al-Quds Force units are operating with Assad's troops. Indeed, some are convinced that the Iranians are probably directing the regime's military forces, because Tehran cannot afford to lose Syria, its key Arab ally.

Syria is Iran's strategic gateway into the Levant and the arms corridor to Hezbollah in Lebanon, as well as the eastern Mediterranean with its newfound energy riches.

The Al-Quds Force has long been based in Syria and neighboring Lebanon where the IRGC helped form Hezbollah in the early 1980s and to all intents and purposes now direct their military operations, principally against Israel.

"Syria's chemical weapons might end up being used not only as a deterrent to prevent outside intervention but could be leveraged by Damascus to help negotiate a settlement allowing elements of the regime to remain in place," Oxford Analytica observed.

Washington is reported to have made contingency plans to send Special Forces teams into Syria to secure the chemical weapons facilities there to protect or destroy the stockpiles if it looks like Assad's regime is losing control.

In July, Damascus acknowledged for the first time it has a large supply of chemical weapons and the precursors required to produce them.

Syria isn't a signatory of the 1993 Chemical Weapons Convention banning the production, stockpiling and use of chemical weapons.

Foreign Ministry spokesman Jihad Makdissi said July 23 the army wouldn't use chemical weapons against the rebels but could use them against outside forces that intervene against the regime.

The Syrian buildup began in the 1970s with help from Egypt, which used poison gas against royalist forces in the Yemen civil war of the 1960s and later from the Soviet Union.

Iran has provided assistance, mainly since 2005, the independent Nuclear Threat Initiative says.

The problem for the Americans and their partners is that the extent and exact locations of the Syrian stockpile aren't known. However, they've presumably gleaned intelligence from a key defector, Maj. Gen. Adnan Nawras Salou, a Sunni who headed Syria's chemical warfare organization until 2008.

U.S. officials say Syria possesses hundreds of tons of chemical agents, particularly the deadly nerve gases sarin and VX, as well as mustard gas, first used in World War I.

U.S. reports indicate the key production and storage centers for nerve gas and mustard gas are concentrated around Safira, Damascus, and the cities of Latakia, a major naval base on the Mediterranean where Russia has a facility, Hama and Homs.

The Syrians have several hundred Soviet-era Scud-C and Scud-D ballistic missiles, as well as some Iranian-designed weapons, as well as bombs and artillery shells capable of carrying chemical warheads.