"Seven members of the National Defence Forces, a pro-regime militia, were killed in clashes with an IS sleeper cell" in the eastern province of Deir Ezzor, said the Syrian Observatory for Human Rights.
ISIS gunmen wounded several other militia fighters, meaning the death toll may climb, it added.
According to AFP the attack came as the militia fighters were sweeping the town of Al-Shola for jihadist remnants, according to the monitor.
The region, near the border with Iraq, was formerly a stronghold of the jihadist group.
Last week, the Observatory said ISIS gunmen in central Syria had ambushed a bus carrying government soldiers in the Wadi al-Azib area of Hama province.
The ambush late Sunday resulted in the deaths of eight soldiers, four allied fighters and three civilians, the Observatory said.
Last month, ISIS said it was behind a December 30 bus ambush in Deir Ezzor province, which killed at least 37 Syrian soldiers.
ISIS in 2014 overran large parts of Syria and Iraq and proclaimed a cross-border "caliphate" before multiple offensives in the two countries led to its territorial defeat.
The group was overcome in Syria in March 2019, but sleeper cells continue to launch attacks in the vast Badia desert running from central Syria eastwards to the border with Iraq.
The Observatory says ISIS attacks have since March 2019 killed more than 1,100 pro-Damascus fighters, including Syrian troops and foreign paramilitaries.
More than 387,000 people have been killed and millions forced from their homes since Syria's civil war broke out in 2011.
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