BEIRUT (AP) — 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.

He accuses the Kurdish People's Protections Units, or YPG, of carrying out "harassing actions" along the border.

Turkish troops have bombarded areas under the control of Syria's main Kurdish military, the People's Protection Units or YPG, multiple times in the past.

In this instance, Ankara appears to be worried that Kurdish fighters might reach Azaz, which is home to a major border crossing point that has been controlled by militants since 2012.

Both the Kurds and Syrian troops have advanced toward Azaz in separate offensives in the area. In addition to sealing the Turkish border, Syrian troops are trying to encircle rebel-held parts of Aleppo, Syria's largest city. If they are able to do so, it will be the biggest defeat for insurgents since the conflict began in March 2011.

Opposition activists reported heavy Russian and Syrian army airstrikes on the villages of Hayan, Anadan and Hreitan north of Aleppo.

The developments come after the United States and Russia announced a plan to halt the violence within a week, but prospects for that happening appear dim.