Fierce clashes gripped the eastern Syrian city of Deir Ezzor on Wednesday, a monitor said, as the Islamic State group set tyres ablaze to create a smokescreen from regime warplanes.

An activist group publishing news on the besieged city told AFP that IS had executed 10 Syrian regime fighters captured during the jihadist onslaught.

IS launched the brutal assault on Saturday to capture the government-held third of Deir Ezzor city, as well as the adjacent military airport.

More than 150 people are reported to have been killed in the fighting.

Syrian and Russian warplanes have carried out intense bombing raids in a bid to halt the jihadist advance, according to a military source and the Syrian Observatory for Human Rights.

IS on Tuesday tried to generate a smokescreen to shield its positions from fresh air strikes.

"The Islamic State group set tyres and barrels of crude oil on fire in Deir Ezzor... to block Syrian and Russian jets heavily bombing the city," the Observatory said.

Since Saturday, 46 regime fighters and 75 IS militants have been killed, along with more than 30 civilians, according to the Britain-based monitor.

A local activist group told AFP on Wednesday that IS had killed at least 10 Syrian government fighters captured during the clashes.

"IS executed them last night by driving over them with tanks," said Omar Abu Leila of Deir Ezzor 24, which publishes news on the eastern city.

He said the gruesome killings took place inside IS-controlled neighbourhoods of the city.

"If IS seizes regime-held neighbourhoods, it could carry out massacres. This is a huge source of concern for us," he said.

The jihadist group, excluded from a nearly three-week ceasefire in Syria, has carried out mass killings of military rivals and civilians, often with particularly grisly methods.

As it advanced on ancient city Palmyra in 2015, it killed dozens of civilians and then staged mass executions of government troops in the city's theatre.

The World Food Programme on Tuesday said the clashes had forced it to suspend aid drops to Deir Ezzor, where more than 100,000 people have lived under IS siege since early 2015.

The WFP has been dropping humanitarian aid into Deir Ezzor since April 2016, and the government-held area is the only place in Syria where the agency has permission for the drops.