The Britain-based Syrian Observatory for Human Rights said IS had captured the village of Bashkun after clashes with Hayat Tahrir al-Sham, a force dominated by a former Al-Qaeda affiliate.
The capture comes after days of fighting between IS and HTS in neighbouring Hama province, during which IS captured a string of villages in the northeast of the region, the Observatory said.
The capture of Bashkun puts IS back in Idlib nearly four years after it was first expelled from the province in northwestern Syria after battles with rival jihadists and rebel groups.
It now holds just a few patches of territory in Syria, and on Saturday Iraq's prime minister declared the war against the jihadist group in his country was now over.
More than 340,000 people have been killed in Syria's multi-faceted war since the conflict began with anti- government protests in March 2011.
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