Two Syrian rebel groups in the town of Azaz have agreed a ceasefire.
The Islamic State in Iraq and Syria (Isis), linked to al-Qaeda, seized the northern town on Wednesday from the larger Western-backed Free Syrian Army.
The fighting has raised fears of a possible war within a war.
The clashes come ahead of one-week deadline for Syria to provide a list of its chemical weapons facilities, as part of a US-Russian deal for the country to destroy its deadly arsenal, reports the BBC.
Russia’s Deputy Foreign Minister, Sergei Ryabkov, is holding talks in Damascus over the deal.
US Secretary of State John Kerry said a “definitive” UN report had proved that the Syrian government was behind a deadly chemical weapons attack in the Damascus suburbs of Ghouta on 21 August.
Damascus – backed by Moscow – insists that rebel forces carried out the attack.
The West wants the deal on ridding the Syrian government of its chemical arsenal enshrined in a UN resolution backed by the threat of military force.
Kerry said the UN Security Council must be willing to act on this when the UN General Assembly holds its annual meeting in New York next week.
But Russia objects to any mention of the threat of force in a UN resolution.
Syria’s President Bashar al-Assad, in an interview with Fox News, said it could take about a year to destroy Syria’s chemical stockpiles and could cost about $1bn (?623m).
