An Islamic State (ISIS) group attack Saturday in the eastern Syrian city of Deir Ezzor killed at least 85 civilians and 50 regime forces, a monitor said, according to AFP.

Syria’s state news agency SANA denounced the attack as a “massacre”, saying around 300 civilians” were killed in the onslaught.

If confirmed it would be one of the highest tolls for a single day in Syria’s nearly five-year war.

The bloodshed in Deir Ezzor came as regime forces battled ISIS in the northern province of Aleppo, killing at least 16 jihadists, and as air strikes hit the ISIS stronghold of Raqa.

The Syrian Observatory for Human Rights said ISIS had advanced into the northern tip of Deir Ezzor city and captured the northern suburb of Al-Baghaliyeh.

Initially it reported that 35 Syrian soldiers and allied militiamen were killed in the multi-front attack, which including a suicide bombing.

But as the day unfolded the death toll rose, with the Britain-based monitor saying that civilians were among those killed in Deir Ezzor.

It said most of the victims were killed execution-style in Al-Baghaliyeh.

Quoting “local sources”, SANA denounced a “massacre”.

“The Daesh (ISIS) terrorists carried out a massacre in Al-Baghaliyeh, claiming the lives of around 300 civilians, most of them women, children and elderly people,” the agency said, according to AFP.

It quoted Syrian Prime Minister Wael al-Halaqi as saying that the “legal and moral responsibility for this barbaric and cowardly massacre… lies on the shoulders of all the states that support terrorism and that fund and arm takfiri (Sunni extremist)” groups.

According to the Observatory, the advance puts ISIS in control of around 60 percent of Deir Ezzor city, capital of the province of the same name in an oil-rich region bordering Iraq.

ISIS said its fighters carried out several suicide bombings against regime forces in Deir Ezzor and seized control of Al-Baghaliyeh and other areas.

The Observatory said Russian warplanes were carrying out heavy air strikes in support of regime forces as they sought to repel the jihadists.

Meanwhile, elsewhere in Syria, regime troops were locked in fierce clashes with ISIS in Aleppo province, with at least 16 jihadists killed after a failed attack on a government position near the town of Al-Bab, the monitor said.

State television also reported that regime forces had repelled an assault in the town.

The Observatory said heavy fighting was ongoing Saturday in the area, with Russian warplanes carrying out strikes in the region between the regime-held Kweyris air base and Al-Bab.

The regime has advanced towards the town, an ISIS bastion, in recent days, and is now within 10 kilometers (six miles) of it, said the Observatory.

That is the closest regime forces have come to Al-Bab since 2012.

Syria’s civil war has killed more than 260,000 people and forced millions to flee their homes.

AFP contributed to this report.