At least 45 Islamic State militants and 20 Kurdish fighters and security officers were killed on Saturday after the extremist group launched a widescale offensive against the Kurds, the Syrian Observatory for Human Rights says.
The Kurdish People's Protection Units (YPG) said that they were clashing with the extremists in 15 positions near the town of Tel Abyad on the Turkish border.
The attack came on the first day of a partial ceasefire, backed by Russia and the US, which excludes fighting against Islamic State and other UN-designated terrorist organizations.
The ceasefire was largely holding in areas of the country where it applied, Observatory director Rami Abdel-Rahman said.
Exchanges of shelling were reported between government and rebel forces in eastern Damascus, while the Observatory said government forces dropped five barrel bombs near the rebel-held city of Jisr al-Shughur in the north-west.
Fighting between government and rebel forces, including Syrian al-Qaeda affiliate the al-Nusra Front which is excluded from the ceasefire, took place in the mountains of north-western Syria, the Observatory said.
Moscow said it was halting operations against rebel forces who had signed up to the truce and would not carry out any air missions on Saturday in order to avoid mistakes.