"The Islamic State group easily took control of the village of Maheen, southeast of Homs, after two suicide attacks," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.
Maheen lies 70 kilometres (43 miles) southeast of the government-controlled provincial capital Homs city, and 35 kilometres east of the Syrian-Lebanese border.
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For the past two years, a ceasefire between rebel factions in the town and regime troops at surrounding checkpoints had governed Maheen. But on Sunday the rebel factions turned against the government fighters and joined ranks with IS militants, Abdel Rahman said.
IS launched its assault from the nearby Christian village of Al-Qaryatain, which it seized in August, he added.
From Maheen, the extremists pushed northeast toward the Christian-majority village of Sadad and the nearby highway running south from Homs to the Syrian capital.
A military source said clashes broke out between regime forces and extremists around Sadad and lasted two hours before subsiding as the Syrian troops pulled out of the village.
The Syrian army, backed by Russian air cover, had been preparing for an imminent attack on the IS-held ancient city of Palmyra further east, but that the takeover of Maheen had set them back.
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Also on Sunday, at least 18 IS fighters were killed in the town of Harbil, north of Syria's second city Aleppo, in clashes with other rebel groups and air strikes by a US-led coalition, Abdel Rahman said.
IS controls eastern parts of Aleppo province and has sought to advance against other rebel groups in the province's west.
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