The Syrian army has tested a chemical weapons delivery system, firing shells at a research centre in its northwestern desert region, the German magazine Der Spiegel reported Monday citing "witnesses".

"Five or six empty shells devised for delivering chemical agents were fired by tanks and aircraft, at a site called Diraiham in the desert near the village of Khanasir," east of the city of Aleppo, the periodical reported.

The Safira research centre in question is regarded as Syria's largest testing site for chemical weapons. It is officially referred to as a "scientific research centre."

Iranian officers, believed to be members of the Revolutionary Guards, were flown in by helicopter for the testing, according to the witness statements cited.

Scientists from Iran and North Korea are said to work in the expansive, fenced-off complex. According to Western intelligence agencies, they produce deadly chemical agents such as sarin and mustard gas.

Last month French President Francois Hollande warned that the use of chemical weapons by the Syrian regime would be a legitimate reason for a foreign intervention.

Washington is also grappling with an array of hellish scenarios that could emerge as the civil war in Syria rages, with the Damascus regime possibly losing control of its chemical arsenal.

Syria's chemical weapons stockpile, which dates back to the 1970s, is the biggest in the Middle East, but its precise scope remains unclear, according to analysts.

Damascus has said it might use its chemical weapons if attacked by outsiders, although not against its own people.