SYRIA accused Islamic State (Isis) terrorists yesterday of using chemical weapons against the besieged eastern city of Deir Ezzor.

Military sources in the field told the national Sana news agency that Isis forces surrounding the city had fired artillery shells containing a “poisonous gas” at their positions on Monday.

The Wahhabi fanatics used 17 car bombs during an assault on the city’s airport, all of which were destroyed by defending troops. During the attack the chemical shells were fired at troop positions in the airport.

Military sources told Al-Masdar News that the shells contained mustard gas, the chemical weapon used extensively in the first world war and since. It is not the first time Isis has been accused of using such weapons.

Deir Ezzor, home to 200,000 people, has been besieged by Isis and other forces for two years. The city, on the Euphrates river near Iraq, relies on air drops by Syrian and Russian transport planes for food, medicine and ammunition.

In southern Aleppo province, the Syrian army and allied militias launched a counteroffensive against Western-backed militants allied with al-Qaida’s Syrian affiliate. Ahrar as-Sham, part of the Saudi-convened High Negotiations Committee to the Geneva Peace talks, broke the Russian-US brokered ceasefire on Saturday when it attacked the town of Tal al-Eiss.

The army, backed by Hezbollah fighters and other allies, are taking part in the offensive to liberate the town — hotly contested since last autumn.

The militants shot down a Syrian air force ground-attack jet with a surface-to-air missile, the second such attack in weeks, raising suspicions that their foreign backers had used the lull in fighting to smuggle sophisticated arms into Syria.

The Sana news agency said attempts were being made to rescue the pilot, but online video showed his capture by insurgents.

In the Sheikh Maqsoud district of Aleppo city, held by the Kurdish YPG militia, rebel shelling killed at least 14 civilians and injured more than 50.