Fierce clashes raged late on Thursday in the majority Kurdish northern Syrian city of Ras al-Ain, on the Turkish border, pitting jihadists against Kurdish fighters, residents and an activist told AFP.
The fighting comes six months after troops loyal to President Bashar al-Assad withdrew from majority Kurdish areas, leaving residents to fend for themselves.
Jihadists have since staged several assaults on the strategic city, and most of its residents have fled.
Syria's Kurds are divided over the revolt against Assad. Some support the regime, others back the revolt, and still others seek to remain neutral.
On Thursday, "the fighting became more intense in the evening after Kurdish fighters received reinforcements to try to stop the fiercest rebel assault ever since insurgents first arrived in the city" in November, a resident identifying himself only as Mohammed told AFP.
The rebels are loyal to the jihadist Al-Nusra Front, which is listed by the United States as a "terrorist" organization, as well as Islamist groups Ghuraba al-Sham and Ahfad al-Rasul, said Mohammed.
While earlier reports said no jihadists were involved in the fighting, an activist from Ras al-Ain—a Kurdish opponent of Assad—confirmed Mohammed's account.
"Armed groups loyal to Al-Nusra Front crossed the Turkish border with three tanks into the city of Ras al-Ain," the activist, who identified himself as Havidar, told AFP in Beirut via the Internet.
While Turkey supports the revolt against Assad, it is also home to a sizeable Kurdish minority that has suffered much persecution and suppression.
Activists say they fear Turkey may be using jihadists in Syria to fight its own battle against the Kurds.
"The advancing rebels did not use the tanks to fight the regime. Instead, they used them to shell Ras al-Ain," said Havidar.
Analysts and activists have voiced fears over the potential consequences should fighting between Kurdish militia and jihadists continue.
"We are concerned about continued clashes in Ras al-Ain between Kurdish militia and rebel fighters from Al-Nusra Front and Ghuraba al-Sham," said prominent Kurdish activist and journalist Massoud Akko.
"Should the fight morph into a struggle between Kurds and Arabs... Syria and the revolt [against Assad] are both in real danger," said Akko.
"We the Kurds have no problem with the [mainstream rebel] Free Syrian Army, so long as it is fighting the regime, but we see no justification for the assault on Ras al-Ain."