The Syrian army is believed to have tested missile systems for poison gas shells at the end of August, the German magazine Der Spiegel reported on Monday.

The report was based on statements from various witnesses indicate.

The tests took place near a chemical weapons research center at Safira east of Aleppo, witnesses told Der Spiegel. A total of 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, the witnesses added.

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

The Safira research center is regarded as Syria’s largest testing site for chemical weapons. It is officially referred to as a “scientific research center.”

Scientists from Iran and North Korea are said to work in the expansive, fenced-off complex, Der Spiegel reported. According to Western intelligence agencies, they produce chemical agents such as sarin, tabun and mustard gas and test them on animals.

In recent months, the guards have been replaced and reinforced by more than 100 elite troops from the 4th Tank Division. In addition, power generators and large supplies of diesel have recently been brought to the plant to safeguard the supply of electricity in the event of an attack by rebels, reports say.

A former army officer who deserted and joined the Free Syrian Army told Der Spiegel, however, that the rebels don’t plan to take the site.

“We hope American troops will secure the plant,” he said. “We don’t want the regime to be able to use the weapons, but neither do we want them to fall into the hands of radicals after the downfall (of the regime).”

Syria is believed to have one of the world’s largest arsenals of chemical weapons. Earlier this month it was reported that Syrian President Bashar al-Assad’s regime has begun moving chemical agents in the country’s arsenal to the port city of Tartus.

According to diplomatic sources quoted in an interview published in the Kuwaiti Al-Rai newspaper, the regime is attempting to create a “safe zone” for top leaders and their families. It is not clear, however, which chemical agents, or what amount, were moved.

Syria has admitted it has chemical weapons and has threatened to use them if attacked by external forces. It claimed it will not use these weapons on rebels fighting to oust Assad.

Recent reports from Syria indicated that Assad has transferred a battery of advanced missiles to the al-Masna border crossing, which is the central route used to transfer equipment and weapons to Hizbullah. Members of the Syrian opposition said that one of two major chemical arsenals of the Syrian regime is located near that border crossing.

Western spy agencies suspect the Syrian government has dispersed several hundred tons of chemical weapons and precursor components across as many as 20 sites across the country, according to a recent report by The Washington Post.

The sites are being monitored, but officials told the newspaper there is growing fear that the monitors have not identified every location and that some of the poisons could be stolen or used by Syrian troops against civilians.