The push on Deir Hafer comes as US aircraft ferried Syrian Kurdish fighters and allies behind IS lines to spearhead a major ground assault on the strategic IS-held town of Tabqa in Raqqa province, which borders Aleppo.
That airlift marked a deepening US involvement in Syria's conflict ahead of a looming battle for IS's de facto capital, the city of Raqqa.
The Syrian Central Military Media says Syrian troops launched the Deir Hafer siege late yesterday after capturing nearby areas.
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