Syria tested firing systems for poison gas shells at the country's largest chemical weapons research center at Safira, east of Aleppo, last month, German weekly Der Spiegel reported on Monday, citing statements from various witnesses.

According to the report, Iranian Revolutionary Guard officers were flown in by helicopter to witness the testing.

Several empty shells, designed to carry chemical weapons, were fired by tanks and helicopters in a desert location near the research center, Der Spiegel quoted the witnesses as saying.

According to the report, Syria has reinforced security at the facility and made efforts to safeguard its electricity supply in recent months, in case of rebel attack.

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