Turkish forces shelled Kurdish fighters in northern Syria for a third day on Monday but failed to stop their advance, the Syrian Observatory for Human Rights said.
The Britain-based monitor said Turkish troops were shelling the road to the west of the town of Tal Rifaat, which has been a key target of a Kurdish-Arab alliance called the Syrian Democratic Forces (SDF), in Aleppo province.
The group said the shelling also targeted the region to the west of the Syrian border town of Azaz.
The alliance's advances have angered opposition-backer Turkey, which accuses the Kurdish forces of links to the outlawed Kurdistan Workers Party that has waged a decades-long insurgency against the Turkish state.
"Fighting has been ongoing since Sunday in the western part of Tal Rifaat between the SDF and rebels," said Observatory director Rami Abdel Rahman.
"At least 26 rebels have been killed in the fighting" for the town in recent days, he added.
Tal Rifaat is a key remaining bastion of mostly Islamist rebel forces in Aleppo province, where the regime this month launched a major operation backed by Russian warplanes.
Simultaneously, the SDF forces have been advancing in the region, seizing the Minnigh air base from rebels, attacking Tal Rifaat and cutting the road leading from the town to Azaz on the Turkish border.
On Monday, SDF forces seized the village of Kfarnaya, two kilometres (1.2 miles) south of Tal Rifaat, the Observatory said.
The Kurdish advance is a complication for Washington, which has backed the SDF in its fight against the Islamic State group, but has also warned the alliance not to take advantage of the fluid situation in Aleppo by seizing new territory.