The US-led coalition fighting the jihadists said it or its allies may have carried out air raids in the area, and it was investigating the alleged civilian deaths.

The raid late Thursday on an ice factory near the village of Al-Soussa close to the Iraqi border killed 28 civilians and 26 IS jihadists, the Britain-based Syrian Observatory for Human Rights said.

It was not immediately clear if the raid on the eastern Deir Ezzor province was carried out by an Iraqi plane or the coalition, according to the monitor, which relies on a network of sources inside Syria.

In a written statement, the US-led alliance said: "The coalition or our partner forces may have conducted strikes in the vicinity of Al-Soussa and Baghour Fukhani" on Thursday.

"We are forwarding the report to our Civilian Casualty Cell for further assessment on this allegation," it said.

Iraqi warplanes have recently carried out strikes against IS in eastern Syria, while coalition aircraft have been supporting Kurdish-led fighters battling the jihadists.

The IS fighters were Syrians and Iraqis, the Observatory said.

Syrian state news agency SANA reported the strike late Thursday, saying more than 30 civilians were killed and accusing the coalition of carrying it out.

The foreign ministry in Damascus decried the coalition for "only succeeding in killing innocent Syrians and destroying Syrian infrastructure", in a statement carried by the outlet.

- IS holdouts -

IS fighters swept across Syria and Iraq in 2014, declaring a cross-border "caliphate" in areas they controlled.

They have since lost most of that territory to various offensives, but still retain pockets of land in Syria including in the country's vast Badiya desert and in Deir Ezzor.

IS fighters have faced two separate offensives in Deir Ezzor on either side of the Euphrates River that cuts through the province.

Russia-supported regime forces have pushed back the jihadists on the western side of the Euphrates, while the US-backed Syrian Democratic Forces have battled them to its east.

Al-Soussa lies to the east of the Euphrates River, in a pocket of territory still held by the jihadists.

IS fighters have been expelled from most urban centres in Syria, but analysts say they have retained their ability to pounce from the desert.

Last month, an IS incursion into the town of Albu Kamal on the west of the Euphrates left dozens of pro-regime fighters dead.

Attacks spiked after the jihadists were evacuated from their last bastion near Damascus in May, many heading to the Badiya desert, the Observatory said.

IS also has a presence in the northwestern province of Idlib, as well as in the southwestern province of Daraa where it has been battling opposition fighters in recent days.

The government and rebels last week announced a ceasefire agreement as the regime moves to retake control of the whole of the province, but IS is excluded from that deal.

The jihadists hold a corner of territory in Daraa on the border with Jordan and the Israeli-occupied Golan Heights.

Overnight Wednesday-Thursday, they seized the nearby village of Heet from rebels who had agreed to hand over their heavy weapons to the regime after deadly clashes.