The Syrian government has been accused of using chlorine gas against the Islamic State of the Iraq and the Levant (ISIL), the latest allegations of chemical weapons use in the country's bitter conflict.

The Syrian Observatory for Human Rights said on Saturday that ISIL fighters showed the effects of a chlorine gas attack as government forces repelled an offensive on a key air base in the country's east.

The UK-based watchdog, which opposes the government, said some the groups fighters had breathing problems as they attempted to close in on the strategic Deir Ezzor military airport.

Syrian State TV quoted an unnamed military official as saying troops repelled attempts by "terrorists" on several areas near the city of Deir Ezzor and killed "tens of them and destroyed their vehicles and weapons."

The station later aired footage of Syrian troops standing near bodies of dead fighters it said were ISIL members who were killed.

SOHR said since ISIL launched their assault on Thursday, 51 of Assad's forces and 68 ISIL fighters had been killed, including two French citizens.

One of the last government-held areas in Deir Ezzor province, the outpost is used by the regime to mount raids on ISIL positions in several areas across the country.

ISIL controls most of Deir Ezzor province, but half the provincial capital remains in government hands.

The oil-rich province lies between ISIL-controlled Raqqa province and the border with Iraq, and is a key prize for ISIL which declared an widely unrecognised Islamic "caliphate" straddling the two countries in June.