The Turkish-backed fighters were locked in a second day of fighting on Tuesday with units from the Syrian Democratic Forces (SDF), a US-backed Syrian coalition of Kurdish and Arab groups, around the village of Ain Daqna.
“Since Monday, 15 fighters from Syrian rebel factions were killed in the clashes and four SDF fighters were wounded, including one in critical condition,” the Syrian Observatory for Human Rights monitor said on Tuesday.
Ain Daqna, in Aleppo province, has been held by the US-backed SDF since February.
It lies in a contested sliver of land between the alliance and Turkish-backed rebels.
In a statement released on Monday, rebels fighting under the banner of “Ahl al-Diyar” said they attacked Ain Daqna because they see the SDF as “occupiers”.
“We promise our people more flash attacks … We will make them [SDF] regret occupying this land and displacing thousands,” the statement said.
Turkey has backed rebels in Syria’s north to take on both the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) as well as the Kurdish People’s Protection Units (YPG), which make up the bulk of the SDF.
Turkey considers the YPG a terrorist group.
Military analyst and retired Jordanian Air Force general Mamoun Abu Nowar said the aim of the Turkish-backed fighters in this particular operation is to isolate the area around the northern city of Afrin and to expand its buffer zone to 5,000sq km.
“Turkey would also like to get Idlib through the process of de-escalation zones,” he told Al Jazeera, from London.
“They have some green light to do some minor operations from Russia, I think, on that regard.”
Al Jazeera’s Mohammed Adow, reporting from Gaziantep at Turkey’s border with Syria, said that the mounting tensions between the two US allies in northwest Syria threatens to open a new front in the country’s complex war.
“For now, the Kurds are still counting on American support, but what they really want, they say, is the establishment of a new Syrian state in which they have a say,” said our correspondent.
Syria’s conflict erupted in March 2011 with anti-government protests, but it has since evolved into a complex civil war drawing in regional powers.
Rights groups including Amnesty International have accused the YPG of razing villages and displacing residents in northern Syria, which the militia has denied.
Suicide bomb kills four
Meanwhile, a suicide car bomb attack killed four people overnight at a Kurdish security checkpoint in northeast Syria, the Syrian Observatory for Human Rights monitor said on Tuesday.
The Britain-based monitor said the blast hit a checkpoint manned by the Asayesh security forces in Hasakeh province, around 20 kilometres from the town of Ras al-Ayn.
At least two of the dead were Asayesh members, the monitor said, while the identities of the other two were not yet confirmed.
Syrian state television also reported the blast on Tuesday and said four people were killed.
Kurdish-controlled areas have come under regular bomb attack, with ISIL often claiming responsibility. There was no immediate claim for Tuesday’s incident.