FILE PHOTO - Iran's President Hassan Rouhani together with his counterparts, Russia's Vladimir Putin and Turkey's Tayyip Erdogan, attend a joint news conference following their meeting in Sochi, Russia November 22, 2017. Sputnik/Mikhail Klimentyev/Kremlin via REUTERS

ANKARA (Reuters) - 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 last month launched an offensive 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.

After the phone call, the Kremlin said in a statement that Putin and Erdogan agreed to strengthen coordination between the two countries’ military and security services in Syria in the fight against terrorism.

(This version of the story fixes day in lead, second paragraph)