BEIRUT: The militant Islamic State (IS) group abducted at least 400 civilians including women and children after seizing territory in a deadly assault on Syria's eastern city of Deir Ezzor, a monitor said Sunday.

“After their attack on Deir Ezzor (on Saturday), IS abducted at least 400 civilians from the residents of the Al-Baghaliyeh suburb it captured and adjacent areas in the northwest of the city,” the Syrian Observatory for Human Rights said.

“Those abducted include women, children and family members of pro-regime fighters,” Observatory chief Rami Abdel Rahman said.

They were transported to areas under IS control in the west of Deir Ezzor province and to the border with the Raqa province to the northwest, he added.

The IS attack on Deir Ezzor on Saturday killed at least 85 civilians and 50 regime forces, the monitor said.

Syria's state news agency SANA, quoting residents, said “around 300 civilians” were killed in the onslaught and denounced it as a “massacre”.

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

IS has in the past carried out mass executions of civilians, including killing hundreds of members of the Shaitat tribe in Deir Ezzor in 2014 after they opposed the militant group.

The group also massacred some 200 Syrian soldiers in August 2014 when it overran the Tabqa military base in Raqa province.

According to the Observatory, the advance puts IS in control of around 60 percent of Deir Ezzor city, capital of the oil-rich province of the same name, which borders Iraq.

Sporadic fighting between IS and regime forces continued on Sunday in the northwest of the city, the monitor said.

Russian warplanes carried out air strikes in support of regime fighters on Al-Baghaliyeh overnight, it said.

The regime still controls parts of the provincial capital and a nearby military airport despite repeated attacks from IS.

The IS assault on Deir Ezzor came as the regime sought to advance in northern Aleppo province, capitalising on a Russian air campaign that began on September 30.

Raqqa, the self-declared capital of IS, has come under frequent air strikes by a US-led coalition, the Syrian air force and Russian warplanes.

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