DAMASCUS/ANKARA — The leaders of Turkey, Russia and Iran agreed on Thursday to meet in Istanbul to discuss the conflict in Syria, a Turkish presidential source said.

Turkish President Tayyip Erdogan discussed the meeting in two phone calls on Thursday with the Russian and Iranian presidents, the source said. The date of the summit would be set in coming weeks.

The three countries have worked together in recent months to try to reduce violence in Syria, even though they have backed rival sides in the nearly seven-year civil war and remain deeply involved in the conflict.

Iran-backed militias and Russian air power have supported a Syrian army offensive in Syria’s northwestern province of Idlib since November, and Turkish forces launched an offensive last month in northern Syria’s Kurdish region of Afrin.

On Monday, Iran urged Turkey to halt the Afrin operation, saying it breached Syrian sovereignty and would increase tension. It was not immediately clear whether Erdogan and Rouhani discussed Afrin in their telephone call on Thursday.

Erdogan and Putin also agreed to speed up the establishment of military observation posts in Syria’s Idlib region under an accord reached by Ankara, Tehran and Moscow last year to reduce fighting between President Bashar al-Assad’s forces and rebels.

Assad: Will take back every inch of Syria

On Wednesday, a U.S.-led coalition and its local allies in Syria struck Syrian government forces with air and artillery fire overnight to repel “an unprovoked attack” near the Euphrates, the coalition said on Thursday.

The incident underscores the potential for further conflict in Syria’s oil-rich east, where the U.S.-backed Syrian Democratic Forces (SDF) alliance of Kurdish and Arab militias holds swathes of land after its offensive against ISIS.

Syrian President Bashar al-Assad, who is supported by Russia and Iran, has said he wants to take back every inch of Syria.

The pro-government forces were “likely seeking to seize oilfields in Khusham” east of the Euphrates in Deir al-Zor province, said a U.S. official on condition of anonymity.

The attack was carried out by 500 troops backed by artillery, tanks, multiple-launch rocket systems and mortars, but the coalition and its allies killed more than 100 of them, the official said.

Syrian state television reported that the coalition had caused “dozens of dead and wounded” by bombing pro-government forces. But a commander in the military alliance supporting Assad disputed the death toll, saying seven members of the pro-government forces were killed and 27 injured.

In a letter to the United Nations, Syria’s foreign ministry described the strike as a “war crime” and called for the coalition to be dismantled, Syrian state news agency SANA said. “We demand that the international community condemn this massacre and hold the coalition responsible for it.”

U.S. tells Syria: We’re not seeking conflict

The U.S.-led coalition was set up in 2014 to battle ISIS militants in both Syria and Iraq, who were largely defeated last year. Some 2,000 U.S. forces remain on the ground in Syria, allied to the Kurdish-led SDF alliance, which holds the largest swathe of territory still outside the control of the government.

The Syrian civil war, now entering its eighth year, has killed hundreds of thousands of people and driven more than 11 million from their homes, while drawing in regional countries and global powers supporting client factions on the ground.

For the most part, the U.S.-backed SDF and the government forces backed by Russia and Iran avoided direct confrontation while both were fighting the common ISIS enemy. Moscow and Washington maintain contacts in eastern Syria to prevent unexpected confrontation between forces they support.

Pentagon spokeswoman Dana White stressed the coalition strikes were purely defensive, as she explained the muscular coalition response at a news briefing. Washington was not seeking to fight Syrian forces, she said.

“We are not looking for a conflict with the regime,” White said.

On Thursday, Defense Secretary Jim Mattis called the attack on U.S.-backed forces in Syria overnight “perplexing,” saying he had no idea what motivated the Syrian army to attempt the coordinated strike.

U.S. supports a pause in Syrian violence

In a recent development on Thursday, the U.S. said it backs a call from the United Nations to put violence in Syria on pause for a month in order to deliver humanitarian aid and facilitate the evacuation of 700 civilians.