Opposition activists say Daoud brigade fighters have evacuated their stronghold in Sarmeen in Idlib to join the Islamic State (IS) in Raqqa province, after it declared an Islamic caliphate days ago.
In January, Daoud brigade pledged allegiance to IS before reaching a "partial solution to end the bloodshed in the eastern suburbs of Edlib" through a ceasefire with other rebels and a renewed focus on fighting the government, according to Reuters.
Syrian rebels, including the main Islamist factions, said last week the creation of a caliphate by IS was "null and void", according to AFP.
"We see that the announcement by the rejectionists of a caliphate is null and void, legally and logically," the groups said in a statement, using a pejorative term to refer to the extremist Islamic State.
The rebels said the announcement of a "caliphate" was divisive and a bid to "abort the blessed revolutions in Syria and Iraq".
IS has kidnapped thousands of Syrians, including rebels and opposition activists, and carried out daily summary public executions in areas under its control.
The radical group controls Raqqa province, large areas of oil-rich Deir El-Zor on the Iraq border, and parts of Aleppo in the north.
Translated and edited by The Syrian Observer