Al-Sukhna, some 70 kilometres (45 miles) northeast of the famed ancient city of Palmyra, is the last town on the road to the eastern city of Deir Ezzor, where a government garrison has held out under IS siege since early 2015.
The Syrian Observatory for Human Rights, a Britain-based monitor, said government forces had captured the town after heavy army artillery fire and air strikes by government ally Russia.
There was no official confirmation of the capture from Syria's government.
State news agency SANA said the army had surrounded the town from three sides.
Since May, Syria's army has been conducting a broad military campaign with Russian support to recapture the vast desert that separates the capital Damascus from Deir Ezzor and other towns along the Euphrates Valley.
Already defeated in its Iraqi bastion of Mosul, IS is facing multiple assaults in Syria.
The US-backed Syrian Defence Forces now control more than half of its most important remaining stronghold Raqa.
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