The Britain-based Syrian Observatory for Human Rights said the attacks over the past 24 hours struck in and around the town of Albu Kamal, near the border with Iraq.
IS has largely been defeated since storming across the region in 2014, but still clings to pockets of territory in eastern Syria and maintains the ability to launch deadly attacks.
Albu Kamal was a long-time stronghold of the jihadists that they lost in the face of a Russian-backed offensive at the end of 2017.
The Observatory says the group now holds less than five percent of territory in Syria, mainly pockets in the deserts of eastern and central regions.
But the group managed to launch a surprise strike in the capital Damascus last month that saw them capture the Qadam district.
The Observatory said the death toll among pro-regime fighters in the strike had risen to 116 after more bodies were discovered in the rubble.
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