Al-Qaeda's Syria affiliate Al-Nusra Front has announced a surprise withdrawal from its front lines against its jihadist rival Islamic State in areas along Syria's northern border with Turkey.
It said the decision was being taken to avoid cooperating with a US-Turkish plan to create an IS-free zone in northern Aleppo province, along the Turkish border.
"We announce our withdrawal from our front lines with the 'khawarij' (IS) in northern parts of Aleppo province," the group said in an online statement published Sunday.
It did not specify which parts of the frontline with IS the group would withdraw from.
Al-Nusra is a key opponent of IS in the northern province of Aleppo along with its powerful Islamist ally Ahrar al-Sham.
The group said it was withdrawing in response to a US-Turkish decision late last month to implement an IS-free zone.
"Al-Nusra Front does not see entering this alliance, helping it, or coordinating with it as legitimate," the statement read.
It said the decision to create the IS-free zone was "not a strategic decision... made by the fighting groups, rather its primary goal is Turkey's national security."
Turkey, a key backer of a range of rebel groups in Syria, has agreed with Washington to work on expelling IS from a swathe of territory in northern Aleppo province.
The US-led coalition fighting IS, which now has access to Turkish bases for the first time, will provide air support, but the nature of the ground forces fighting against IS remains unclear.
The Syrian Observatory for Human Rights monitoring group said Al-Nusra had so far fully withdrawn from the villages of Dalha and Harjaleh, which lie near the Turkish border.
"There are other villages that they officially withdrew from, but they stayed in the hands of Nusra's allies," said Observatory head Rami Abdel Rahman.
Aaron Stein, a non-resident fellow at the Atlantic Council research institute, said Al-Nusra's withdrawal could make it easier for Washington to work with ground forces in the would-be IS-free zone.
"The way I'm looking at it is Turkey influenced Ahrar al-Sham, which put pressure on Al-Nusra... to remove itself from the areas proposed in the buffer zone," Stein told AFP.
The Al-Nusra withdrawal makes it "more palatable for the US to give more support to rebel groups there," he added.
Few details have emerged on which armed opposition forces Ankara and Washington will partner with to enforce their IS-free zone.
Even if ground forces don't receive direct military support from the US, Stein said they could benefit from US-led air cover over the border area.
Despite the withdrawal, Al-Nusra will continue to have "influence" and allies -- particularly Ahrar al-Sham -- in the zone in question, Stein said.
Syria's conflict began with peaceful demonstrations in March 2011, but spiralled into a multi-front war that has become increasingly dominated by jihadist forces.