At least 27 Syrian regime forces have been killed in a jihadist assault on a military base in the northern province of Raqa, a monitoring group said on Thursday.
The Syrian Observatory for Human Rights said 11 jihadists from the Islamic State were also killed in the attack on Brigade 93, including three who blew themselves up at the base.
IS fighters now "control major parts of the base," the Observatory said.
Some troops had already pulled out of Brigade 93 in July after IS captured Division 17, another base in the region where at least 85 people were killed in fighting or summarily executed shortly afterwards, it said.
Brigade 93 and Tabqa military airport were the last positions held by the Syrian army in Raqa province.
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Since joining the conflict in Syrian in spring 2013, IS has seized control of almost all of Raqa and Deir Ezzor provinces on the border with Iraq, where it has waged a devastating offensive since June.
In Deir Ezzor, IS has executed 19 men at the Al-Omar oil field which it captured on July 3, the Observatory said.
Most of the men were shot dead, with one of them decapitated, "in one of the bloodiest executions", according to the monitoring group.
The Observatory said the executed men were staff at the oil field, one of Syria's largest, who were taken prisoner when IS took over.
At the end of June, IS declared a caliphate in territory it controls in Syria and Iraq.
The Observatory estimates more than 170,000 people have died in violence in Syria since March 2011, the start of a peaceful uprising against President Bashar al-Assad's regime that developed into an armed revolt.