Islamic State militants launched an assault on a Kurdish-controlled town on Syria's border with Turkey on Saturday, prompting air strikes by the US-led coalition to try to drive them back.
The hardline Sunni fighters attacked Tel Abyad, which is controlled by the Syrian Kurdish YPG militia, and the nearby town of Suluk in the early hours of Saturday, YPG spokesman Redur Xelil and Turkish security sources told Reuters.
Coalition war planes carried out 10 air strikes to try to repel the assault, the Syrian Observatory for Human Rights reported. It said at least 45 Islamic State militants and 20 Kurdish militia fighters had been killed.
The attack was launched hours after a "cessation of hostilities" came into effect under a US-Russian plan, although the temporary truce does not apply to Islamic State and al Qaeda affiliate the Nusra Front, meaning the Syrian government, Moscow and the coalition reserve the right to strike them. The truce appeared largely to be holding across much of Syria.
Xelil said the YPG and Syrian Kurdish internal security forces were able to "crush this attack and encircle the attackers", although he gave no casualty toll.
The YPG captured Tel Abyad from Islamic State last year in an offensive backed by US-led air strikes. The town lies north of Raqqa and had been a key supply line for the jihadi capital.
The Turkish security sources said the attack was launched on two fronts and that the sound of gunfire and explosions, audible from the town of Akcakale on the Turkish side, had continued for several hours. Intermittent gunfire could still be heard.
The security sources and a witness in Akcakale said war planes thought to be from the US-led coalition against Islamic State struck the jihadist positions and that the Turkish army had increased patrols on its side of the border.
While not directly addressing the Tel Abyad attack, 14 Turkish F-16s patrolled the Turkish-Syrian border on Friday, Turkish military said.
Xelil said some of the attackers infiltrated from the Turkish border to the north, reiterating accusations that Turkey was supporting the group. Turkey has consistently denied those accusations and the security sources said recent measures to stop illegal crossings meant it was impossible that the attackers had entered from Turkey.
Other attackers infiltrated from the south, Xelil said. He said dozens of Islamic State fighters had been killed.