Beirut: At least 85 Syrian army troops were killed as the Islamic State of Iraq and the Levant advanced on a regime position in the northern province of Raqa, a monitoring group said Saturday.

The Syrian Observatory for Human Rights said the fate of around 200 other soldiers remained unknown, as the Isil assault forced the army to pull back late on Friday.

The Division 17 base has fallen from army control but the militants have not yet moved into all the site’s buildings “for fear of air strikes”, said the Britain-based group’s director, Rami Abdul Rahman.

The Isil lost at least 28 militant fighters, he said.

The Observatory said more than 50 troops were summarily executed, 19 more were killed in a double suicide attack and at least 16 others had died in the Isil assault launched early Thursday.

“Hundreds of troops surviving withdrew on Friday to safe places - either to nearby villages whose residents oppose Isil or to nearby Brigade 93 - but the fate of some 200 remains unknown,” said Abdul Rahman.

“Some of the executed troops were beheaded, and their bodies and severed heads put on display in Raqa city,” stronghold of the feared Isil, he said.

Video shot by the militants and distributed via YouTube showed the men, apparently inside troops’ former living quarters in Division 17, burning a portrait of President Bashar Al Assad.

One militant behind the camera refers to the army as being composed of “heretics, apostates”, while showing bottles of spirits and cigarettes that the troops had been keeping.

The assault on Division 17 comes less than two weeks after Isil killed 270 security guards, employees and members of the paramilitary National Defence Forces during an assault on a gas field in Homs, central Syria.

On Friday, the UN Commission of Inquiry on Syria said Isil fighters accused of atrocities were expected to be added to a UN list of possible war crime indictees.