Five or six shells capable of delivering chemical weapons were fired by tanks and aircraft during an exercise conducted last month, the German weekly Der Spiegel reported on Monday.

A number of officers believed to be members of Iran’s Revolutionary Guards corps were flown by helicopter to the Safira research centre, regarded as Syria’s largest chemical weapons testing facility, for the exercise, the publication quoted “various witnesses” as saying.

North Korean and Iranian scientists frequently work at the facility, which is used to produce chemical agents such a sarin, tabun and mustard gas, according to claims by Western intelligence agencies cited by Der Spiegel.