Soldiers after the Turkish military and the FSA liberated Afrin in northwestern Syria from the control of Kurdish militia during ongoing Operation Olive Branch on February 19, 2018.

The leaders of the three countries will meet in Ankara for talks on a new constitution for Syria and increasing security in "de-escalation" zones across the country, Turkish officials say.

The Syria summit brings together two powers which have been President Bashar al-Assad's most forceful supporters, Iran and Russia, with one of his strongest opponents, Turkey.

Cooperation between the rival camps raised hopes of stabilising Syria after seven years of conflict in which 500,000 people have been killed and half the population displaced.

But the violence has raged on, highlighting strategic rifts between the three countries who, in the absence of decisive Western intervention, hold Syria's fate largely in their hands.

Syria's army and Iran-backed militias, with Russian air power, have crushed insurgents near Damascus in eastern Ghouta - one of the four mooted "de-escalation zones".

Turkey, which sharply criticised the Ghouta offensive, waged its own military operation to drive Kurdish YPG fighters from the northwestern Syrian region of Afrin. It has pledged to take the town of Tel Rifaat and push further east, angering Iran.

"Whatever the intentions are, Turkey's moves in Syria, whether in Afrin, Tel Rifaat or any other part of Syria, should be halted as soon as possible," a senior Iranian official said.

Iran has been Assad's most supportive ally throughout the conflict. Iran-backed militias first helped his army stem rebel advances and, following Russia's entry into the war in 2015, turn the tide decisively in Assad's favour.

A Turkish official said Ankara will ask Moscow to press Assad to grant more humanitarian access in Ghouta, and to rein in air strikes on rebel-held areas. "We expect ... Russia to control the regime more," the official told reporters this week.