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 the main fighting force of Syrian Kurds and a key ally of the U.S.-led coalition battling the Islamic State group. Turkey, which is also in the alliance, considers it an affiliate of the Kurdistan Workers' Party, or PKK, which has waged a decades-long insurgency against Ankara.
A coalition of Kurdish-led Syrian fighters known as the Syrian Democratic Forces recently seized a number of villages near Turkey's border. Ankara appears concerned they could reach the opposition stronghold of Azaz, which is home to a major border crossing that has been controlled by militants since 2012.
Diplomats from a group of countries that have interests in Syria's five-year 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 where they continue to sleep in the open air, has accelerated.
A top Syrian opposition figure, former Prime Minister Riad Hijab, criticized Russia on Sunday for continuing with its bombing in Syria, and also stopped short of declaring a clear commitment to implement a planned temporary truce.
"You ask me if I accept a cease-fire or a cessation of hostilities. I ask you: Why is the onus on the opposition and whether it has preconditions for negotiations?" Hijab said. "I would like to see a single day of a cessation of hostilities in order to give a chance for real political movement." He was addressing the Munich Security Conference, an annual gathering of foreign and security policy leaders.
On Saturday, President Obama spoke by phone with Russian President Vladimir Putin to call for an end to airstrikes against moderate opposition forces in Syria.
During the call, Obama stressed the need for quick humanitarian access to besieged areas and a nationwide cessation of hostilities, the White House said in a statement released Sunday.
The Kremlin said earlier in an emailed statement that Putin emphasized the importance of a united antiterrorism front and close contacts between Russian and U.S. defense forces.