ALEPPO - Al-Qaeda shot down a government warplane over the town of Al-Eis in northern Syria on Tuesday and captured one crew member alive, a rebel source and a monitoring group said.

The rebel source said it was "likely that (Al-Qaeda's Syria affiliate) Al-Nusra Front shot down the plane and took the pilot," adding that the plane had been hit by heavy machinegun fire.

The Syrian Observatory for Human Rights said Al-Nusra downed the plane, which it said was likely being flown by a Syrian air force pilot. The Al-Qaeda affiliate is not party to the ceasefire between government forces and non-militant rebels brokered by the United States and Russia that has been in place since February 27.

On Friday, Al-Nusra and its allies pushed regime loyalists out of Al-Eis, a strategic town in Aleppo province.

In video footage circulated on social media purporting to show the scene where the plane came down, a dozen men crowd around a man lying in the dirt. Some of them cry: "He's Syrian, he's Syrian!" and others yell: "Get his weapons off him!"

AFP could not confirm the authenticity of the footage. Last month, Islamist rebels shot down a regime warplane over the village of Kafr Nabuda in the central province of Hama.

Meanwhile, the Islamic State group has mounted a deadly gas attack against Syrian troops at a besieged eastern airbase, state news agency SANA said, the latest report of the militants' use of chemical weapons.

SANA did not say precisely how many soldiers had been killed in the attack on the government-controlled airbase outside the divided eastern city of Deir Ezzor.

"Daesh (IS) terrorists attacked Deir Ezzor military airport with rockets carrying mustard gas, causing some people to suffocate," it reported late Monday.

It is the latest in a string of suspected mustard gas attacks by the militants in Syria and neighbouring Iraq.

On March 9, a suspected IS gas attack on the Iraqi town of Taza, south of Kirkuk, killed three children and wounded some 1,500 people, with injuries ranging from burns to rashes and respiratory problems.

While the chemical agents allegedly used by IS so far have been among their least effective weapons, the psychological impact on civilians is considerable. A total of 25,000 people fled their homes in and around Taza last month, fearing another attack.

IS has been battling to capture Deir Ezzor airbase since 2014. It provides the only supply route other than air drops to the government-held sector of the city, where more than 200,000 civilians are living under IS siege.

On Monday, an IS bombardment of two government-held districts of the city killed seven civilians, according to the Syrian Observatory for Human Rights.

Two suicide bombers also blew themselves up in the village of Jafra near the airbase, the Britain-based monitoring group said. Deir Ezzor province is vital for the militants because it lies between their de facto Syrian and Iraqi capitals Raqa and Mosul.

In recent weeks, IS has faced intense pressure in Syria at the hands of both the Russian-backed army and US-backed Kurdish-led rebels.

An offensive by the army pushed the militants out of the ancient city of Palmyra late last month, opening up the possibility of a strike across the desert to relieve the siege of Deir Ezzor.