Syrian rebels fighting the Islamic State of Iraq and the Levant (ISIL) in the north of the country on Wednesday killed a jihadist leader in the town of Saraqeb, a watchdog said.
Meanwhile in jihadist-held Raqa, further east, ISIL set free dozens of rebels it had captured in battles over the past few days, said the Syrian Observatory for Human Rights.
"ISIL's emir in Saraqeb (Idlib province) Abul Baraa was shot dead this morning," said Rami Abdel Rahman, director of the Britain-based monitoring group.
Abul Baraa, a Belgian jihadist of Arab origin, last week warned Syrian rebels to halt an anti-ISIL offensive they launched early January or face suicide attacks against their positions.
Since his January 6 warning, Islamist and moderate rebel positions have been hit by jihadists' suicide attacks -- mostly car bombs -- in Aleppo, Idlib and Raqa provinces, which have killed dozens of opposition fighters.
Abul Baraa's killing on Wednesday came as rival "Islamist rebels infiltrated Saraqeb town and took up positions in a building there. They opened fire" on the emir's convoy and killed him.
Once a bastion of moderate activism against President Bashar al-Assad's regime, Saraqeb was turned into a stronghold of ISIL in recent months.
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An erstwhile ally of Syria's rebels, ISIL has become hated for its quest for hegemony and its systematic abuses against rivals, activists and journalists, among them Westerners.
According to Abdel Rahman, "hundreds of ISIL fighters are still in Saraqeb".
More than 700 jihadist and rebel fighters have been killed in 12 days of combat pitting the two sides against each other.
While rebels have mostly been on the offensive in Aleppo and Idlib provinces, ISIL has regained near-total control of Raqa province.
On Tuesday night, ISIL "set free dozens of Islamist rebels" who had been captured in Raqa during the recent battles in the province.
Earlier that day, rebels fighting the jihadists withdrew from the province.
On Wednesday, the government air force carried out an air raid on Raqa city, the only provincial capital to have fallen out of Assad's control since the start of Syria's revolt in March 2011.
More than 130,000 people have died over the course of Syria's war, and millions more have been displaced.