At least 85 Syrian army troops were killed as Islamic State fighters advanced on a regime position in the northern province of Raqa.
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 last night.
The Division 17 base has fallen from army control but "the jihadists have not moved into the site for fear of air strikes", said the Britain-based group's director, Rami Abdel Rahman.
The IS lost at least 28 fighters, he said.
The Observatory said more than 50 troops were executed.
The assault on Division 17 comes less than two weeks after 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.
The UN Commission of Inquiry on Syria said IS fighters accused of atrocities are expected to be added to a UN list of possible war crime indictees.
Yesterday, a Syrian government helicopter allegedly dropped a barrel bomb on the Sakhour eastern neighborhood, in the northern city of Aleppo.
Dozens of people have been killed in 24 hours of clashes in northern Syria between regime forces and militants.