State media in Syria have accused militants from so-called Islamic State (IS) of using mustard gas against government forces in the north-east.
Troops came under fire at an air base which IS has been trying to capture in Deir al-Zour, they say.
Meanwhile, rebels have shot down a Syrian fighter plane over the province of Aleppo.
Unconfirmed reports say militants from Nusra Front have captured one of the crew alive.
Reuters news agency cites monitoring and rebel sources as saying the pilot was taken by the al-Qaeda-linked group after the plane crashed in countryside.
Syrian state TV said the plane had been shot down by a surface-to-air missile and a rescue operation was under way. It did not say whether the pilot had been captured.
'Chemicals in rockets'
Reporting on Monday night, the Syrian state news agency Sana said, "Daesh [IS] terrorists attacked Deir al-Zour military airport with rockets carrying mustard gas, causing some people to suffocate."
Large parts of Deir al-Zour city containing about 200,000 people have been besieged by IS for months. Earlier this year the UN started dropping aid to civilians trapped in government-held areas.
The air base, south of the city, is the government's last strategic asset in the province, which is mostly under IS control.
Last month IS was suspected of carrying out a mustard gas attack on the northern Iraqi town of Taza, killing three children and wounding hundreds of people.
It is also reported to have used mustard gas against rebels in Marea, northern Syria, and against Kurdish Peshmerga fighters in northern Iraq last August.
Sulphur mustard - commonly known as "mustard gas" although it is liquid at ambient temperature - is a powerful irritant and blistering agent which causes severe damage to the skin, eyes and respiratory system and internal organs.
The Syrian government has itself been repeatedly accused of perpetrating chemical weapons attacks, something it vehemently denies.
Syria is supposed to be free of chemical weapons since June 2014 under a UN-backed deal.