Turkey shelled positions held by the main Kurdish militia in northern Syria for a second day Sunday, adding complexity to an inflamed situation in the area where Russian-backed Syrian government forces are also on the march, opposition activists said.

The Syrian Observatory for Human Rights activist group said two fighters from the Syrian Democratic Forces - a coalition of Kurdish and Arab fighters - have been killed and seven others wounded in the shelling.

There was no immediate confirmation by the group, which is dominated by Kurdish fighters from the People's Protection Units known as the YPG.

The group has seized a number of villages in the northern province of Aleppo near the Turkish border in recent days, and appears poised to move to the border town of Azaz, an opposition stronghold. That has alarmed Turkey, which considers the group to be an affiliate of the Kurdish PKK movement which it considers to be a terrorist organization.

Opposition groups said Saturday that Turkish troops fired artillery shells that targeted the Mannagh air base in Aleppo province, which was captured by Kurdish fighters and their allies earlier this week.

Turkish Prime Minister Ahmet Davutoglu said late Saturday that his country's military fired at Kurdish fighters in northern Syria in response to a provocation along the border. He said Turkish forces retaliated against a Kurdish faction "that presented a threat in Azaz and its environs" in line with the country's rules of engagement.