Islamist rebels have shot down a Syrian government aircraft over Aleppo.
Reports suggest that militants from the Nusra Front have captured the pilot alive.
The Syrian government has confirmed that one of its planes has been shot down by a surface-to-air-missile while on a reconnaissance mission.
A regime search and rescue operation is underway.
In another development the Syrian state news agency SANA is reporting that ISIL fighters have attacked Deir al-Zour military airport with “rockets carrying mustard gas causing some people to suffocate.”
Swathes of Deir al- Zour City in the north east of the country have been besieged by ISIL for months.
The UN has begun a programme of aid drops to the 200,000 people trapped within the city.
The airport is the one remaining strategic asset in the area controlled by Assad’s forces.
It is not the first time accusations of chemical attacks have been reported in Syria.
The government has issued numerous denials that it had used chemical weapons.
Under a United Nations programme Syria is supposed to have been free of chemical weapons since June 2014.