BEIRUT • Terrorists from the Islamic State in Iraq and Syria (ISIS) group have abducted more than 400 Syrian civilians after capturing new ground in a major assault on the city of Deir Ezzor that left dozens dead.

The shock attack comes despite a Russian air campaign targeting the organisation that began in September, and more than a year of strikes by a US-led coalition against the ISIS militants in Syria.

The Britain-based Syrian Observatory for Human Rights said ISIS had killed at least 135 people in the multi-front attack that began last Saturday.

The dead comprised 85 civilians and 50 regime fighters, according to the monitor, which said yesterday that ISIS had also kidnapped more than 400 civilians from captured territory.

"Those abducted, all of whom are Sunnis, include women, children and family members of pro-regime fighters," Observatory chief Rami Abdel Rahman said.

He said they had been taken to areas under ISIS control in the west of Deir Ezzor province and to the border with Raqqa province - the main ISIS stronghold in Syria - to the north-west.

The monitor said at least 42 ISIS fighters had been killed in the attack, adding that fighting was ongoing yesterday, with regime forces backed by Russian air strikes trying to recapture lost ground.

Syria's state news agency Sana said that at least 300 civilians, "most of them women, children and elderly people", had been killed in the assault. It denounced the deaths as a "massacre".

The ISIS assault puts the group in control of around 60 per cent of Deir Ezzor city, which is the capital of the surrounding province of the same name.

If confirmed, the death toll in the assault would be one of the highest in a single attack by ISIS, though the terrorists have carried out mass murders before. In 2014, its fighters killed hundreds of members of the Sunni Shaitat tribe in Deir Ezzor province after they opposed the militants. And in August 2014, the group massacred some 200 Syrian soldiers when it overran the Tabqa military base in Raqqa province.

The militants have carried out mass abductions before, seizing more than 200 civilians from central Homs province in August 2014, and at least 220 Assyrian Christians from villages in the north-east of the country months earlier.

AGENCE FRANCE-PRESSE