As the Islamic State group closes in on government-held areas of Syria's Deir Ezzor, residents said they are terrified of falling victim to the mass killings for which the jihadists have become infamous.

Besieged by IS since early 2015, the regime-controlled third of Deir Ezzor city is home to an estimated 100,000 people.

Since Saturday, IS has steadily advanced in a fresh assault on that part of the city, sparking fears among residents of widespread atrocities.

"Civilians in the city are terrified and anxious, afraid that IS will enter (government-held parts of) the city since they accuse us of being 'regime thugs'," said Abu Nour, 51.

He spoke by phone from inside the city, roughly one kilometre (less than one mile) from approaching IS forces.

Deir Ezzor sits in the oil-rich eastern province of the same name, most of which is controlled by IS.

Abu Nour told AFP that residents were haunted by previous abductions and mass executions carried out by IS in the broader province.

"The way they killed them is stuck in people's minds here," he said.

IS is notorious for using particularly gruesome methods to kill military rivals and civilians alike, including beheading, lighting them on fire, or launching rockets at them from just metres (feet) away.

As the group advanced on ancient city Palmyra in 2015, it killed dozens of civilians, accusing them of being regime loyalists, then staged mass executions of government troops in the city's theatre.

According to one activist group, IS has already begun executing Syrian soldiers it took captive during the clashes in Deir Ezzor.

IS executed 10 soldiers "by driving over them with tanks", said Omar Abu Leila, an activist from Deir Ezzor 24, which publishes news on the city.

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

- 'Hunger will ravage us' -

In its push for Deir Ezzor, the jihadist group has launched salvos of rockets on the neighbourhoods it besieged.

"Shells have rained down on us for five days," Umm Inas, another resident, told AFP by phone.

"There's very little movement in the street because people are afraid of these shells, which spare no one," the 45-year-old said.

She warned the humanitarian situation was getting increasingly dire, after the World Food Programme said on Tuesday it could no longer carry out air drops over the city because of the fighting.

"If the situation continues like this, hunger will ravage us. The air drops were our only lifeline," Umm Inas said.

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.

In the past, government and Russian warplanes have also delivered desperately needed humanitarian aid to the city via air drops.

A medical source in the city told AFP more than 100 civilians had been wounded in the recent fighting, and some were taken north to the Kurdish-majority city of Qamishli.

"Some intractable cases were flown to Qamishli because they need special treatment that isn't available in Deir Ezzor," the source said.