BEIRUT (AP) -- 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.
Akdogan says Kurdish gains in northern Syria - facilitated by a Russian-backed government offensive in the same area - are also putting "unacceptable" pressure on opposition-held areas in Aleppo and the nearby town of Tel Rifaat.
The private Dogan news agency broadcast footage of Turkish howitzers opening fire and shells raising plumes of smoke in Syria. It said the army hit targets in the Mannagh air base and two villages, all controlled by Syrian Kurdish fighters.
The Syrian Observatory for Human Rights, an opposition group that monitors the conflict, said two fighters from the SDF - a coalition of Kurdish and Arab fighters - have been killed and seven others wounded in the shelling. A Turkmen rebel commander, Zakariyya Karsli, told Anadolu Agency Turkish shelling killed at least 29 Kurdish fighters.
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.
France meanwhile called on Syria and Russia to halt their airstrikes and for Turkey to stop shelling Kurdish areas. In a statement, it expressed concern about the "deteriorating situation in Aleppo and northern Syria."
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.
The Observatory and the Lebanon-based A-Mayadeen TV said at least 350 Turkey-backed armed rebels crossed into Syria from Turkey Sunday to shore up rebels fighting Kurds near Tel Rifaat.
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 Assad and has sent weapons, money and military advisers to Syria to help bolster his forces.
Esmaili's remarks came after Turkey and Saudi Arabia -- leading supporters of the rebels battling to topple Assad -- said they were open to sending ground troops into Syria to battle the Islamic State group. Turkey's foreign minister, Mevlut Cavusoglu, also said Saturday that Saudi Arabia is "ready to send both jets and troops" to Turkey's Incirlik air base.
Fighting and airstrikes continued elsewhere in the country Sunday, including in rebel-held suburbs of the capital Damascus. Opposition activists reported Syrian army helicopters dropped around 10 barrel bombs on the town of Daraya. There was no immediate word on casualties.
Soguel reported from Istanbul, Turkey. Associated Press writers Albert Aji in Damascus, Syria, Nasser Karimi in Tehran, Iran, and Lori Hinnant in Paris contributed to this report.