Turkey is seeking to contain the Kurdish YPG group in Syria after it gained control of major areas in northern Syria
Syrian rebels said on Friday they were preparing to join the Turkish military in a major new offensive against Kurdish forces in northwestern Syria, raising the prospect of yet another front in an increasingly complex conflict.
The goal of the battle would be to regain a string of Arab villages in Syria near the Turkish border that were seized last year by Kurdish-led militia fighters, the rebels told Reuters.
"There is a coming extensive joint operation we are preparing with the Turkish army to expel these extremist separatist militias (Kurdish YPG) from our land," Mustafa Sejari, a senior official in the Western and Turkish-backed rebel group Liwa al-Mutasem, said.
Turkey is seeking to contain the Kurdish YPG group in Syria, viewing the force as an extension of Kurdish separatists fighting inside its borders.
Turkey has been pouring tanks, artillery and troops into the Syrian town of Azaz, the last town before the border with Turkey, its Syrian rebel allies say. It marks the biggest deployment of Turkish troops into Syria since Ankara launched a major incursion into northern Syria last year.
The head of the Syrian Kurdish YPG militia told Reuters on Wednesday that Turkish military deployments near Kurdish-held areas of northwestern Syria amounted to a "declaration of war" which could trigger clashes within days.
Turkish President Tayyip Erdogan said on Wednesday that Turkey was ready to carry out ground operations against Kurdish forces in northern Syria along with the rebel forces it backs there if necessary.
There was no further comment from Ankara, or from Washington, on Friday.
The tensions threaten to pitch two US allies - the YPG and NATO-member Turkey - into a conflict against each other that could derail the U.S.-backed assault on Islamic State's base of operations at Raqqa. The YPG is spearheading the attack on Raqqa.
Two rebel commanders said the Turkish forces were now stationed in several garrisons on Azaz's outskirts and further south-east in the town of Marae, that lies on the frontline with Kurdish People's Protection Units (YPG) forces to the west.
The rebels include groups that have been vetted by the US Central Intelligence Agency and Turkish-backed brigades. The main goal of the military operation is to recapture Tel Rifaat, a town south-west of Azaz that the YPG militia captured from rebels in February 2016, they said.
Tel Rifaat and nearby areas including the Menigh air base fell to the YPG as the rebels were trying to fend off a major assault by Syrian government forces backed by the Russian air force and Iranian-backed militias. It was a prelude to the rebels' defeat in eastern Aleppo - their biggest single setback of the war.
The fighting forced at least 150,000 residents of these villages to flee to Azaz. They are currently sheltering in at least five refugee camps at the Turkish border, and securing their return is a major priority for the rebels and Turkey.
Sejari said the rebel forces had held talks with the YPG in the presence of US military commanders to try and avoid a confrontation over the villages, but the negotiations had failed.
A second rebel official confirmed the failure of the US-mediated talks, saying: "There are villages that the YPG cannot keep, these are occupied areas."
A YPG spokesman said he had no comment to make on rebel reports of US mediated talks, and questioned why the YPG would hand over the area.
The rebels preparing to take part in the assault are the same factions that took part in the Turkey-backed operation launched last year to drive Islamic State away from the border and to prevent further expansion of YPG influence.
It also allowed some of the rebels to establish control over a section of the Turkish-Syrian border extending from Azaz to Jarablus on the western bank of the River Euphrates. The area has become a de facto buffer zone safe from Syrian army air strikes, where a Turkey-backed administration is taking shape.
Turkey and the YPG have been firing artillery at each other in recent weeks. Rebels say the YPG has stepped up shelling of Arab villages near Azaz, and say this is the prelude to the planned Turkish operation. The YPG says the Turkish military has been shelling its areas on and off for more than a year.