At least 85 Syrian soldiers were killed, some of them beheaded, as the jihadist Islamic State took over a base 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 IS assault forced the army to pull back late on Friday.
The Division 17 base in Raqa has fallen but the jihadists have not yet moved into all its buildings "for fear of air strikes", said the Britain-based group's director, Rami Abdel Rahman.
The IS lost at least 28 jihadist 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 IS assault launched early Thursday.
"Hundreds of troops surviving withdrew on Friday to safe places -- either to nearby villages whose residents oppose IS or to nearby Brigade 93 -- but the fate of some 200 remains unknown," said Abdel Rahman.
"Some of the executed troops were beheaded, and their bodies and severed heads put on display in Raqa city," stronghold of the feared IS, he said.
Nearly all of Raqa province is now under IS control, except for Brigade 93 and Tabaqa military airport, still held by the army.
An activist in Raqa, Abu Ibrahim, said on Facebook that three army helicopters were being used to transport the surviving troops out of Brigade 93.
Video shot by jihadists and distributed on YouTube, meanwhile, showed IS fighters apparently inside Division 17 living quarters burning a portrait of President Bashar al-Assad.
One jihadist behind the camera shows bottles of alcohol and stashes of cigarettes.
Elsewhere, the IS killed at least 30 troops and pro-regime paramilitary in an overnight ambush in the northern province of Aleppo, the Observatory said.
The flareups come less than two weeks after the IS 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 IS fighters accused of atrocities were expected to be added to a UN list of possible war crime indictees.
Some Western-backed rebels seeking Assad's ouster initially welcomed IS as a potential ally. But the extremist group's abuses and quest for domination has turned the opposition against it.
Syria's war has killed more than 170,000 people since March 2011.