Zeina Karam and Dominique Soguel

Associated Press

Beirut — Turkey shelled positions held by a U.S.-backed Kurdish militia in northern Syria for a second day on Sunday, drawing condemnation from the Syrian government, whose forces are advancing against insurgents in the same area under the cover of Russian airstrikes.

Turkey’s state-run Anadolu Agency said Turkish artillery units fired at Kurdish fighters in the Syrian town of Azaz in Aleppo province, saying it was in response to incoming Kurdish fire.

Turkish troops have shelled areas under the control of Syria’s main Kurdish faction, the People’s Protection Units, known as YPG, in the past. The group has been most effective in the fight against the Islamic State group in Syria, but Ankara appears increasingly uneasy over the group’s recent gains in the country’s north.

“Turkey has responded in this manner in the past,” said Turkey’s Deputy Prime Minister Yalcin Akdogan. “What is different is not that Turkey has responded in such a way but the fact that there are different movements in the region. The YPG crossing west of the Euphrates is Turkey’s red line.”

The YPG is a key ally of the U.S.-led coalition battling the Islamic State group.

Diplomats from a group of countries that have interests in Syria’s civil war, including the U.S., Russia, Turkey, Saudi Arabia and Iran, agreed on Friday to seek a temporary “cessation of hostilities” within a week. But the fighting on the ground, which has sent tens of thousands of people fleeing toward the Turkish border, has accelerated.

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.

The Syrian government condemned Turkey’s shelling of Syrian territory, describing it as an attempt to raise the morale of “terrorist” groups it supports.

Also on Sunday, Iran’s air defense chief said his country is ready to help defend Syria’s airspace, marking the first time Iran has offered to assist with Syrian air defenses.

The semi-official Tasnim news agency quoted Gen. Farzad Esmaili as saying “we will help Syria in a full-fledged manner if the Syrian government requests help.” He said any such aid would be provided in an “advisory” capacity.

Iran is a close ally of Syrian President Bashar al-Assad and has sent weapons, money and military advisers to Syria to help bolster his forces.