Russian President Vladimir Putin met his Turkish counterpart Recep Tayyip Erdogan on Tuesday to discuss the situation in northern Syria, where Turkey has vowed to resume an offensive against Kurdish fighters.

Iran on Monday denounced as "unacceptable" any move by Turkey to establish military bases in Syria, saying such a step would face opposition from the Islamic republic and other countries.

U.S. Vice President Mike Pence met Turkish President Recep Tayyip Erdogan on Thursday to push for a ceasefire in Syria after Ankara rebuffed international pressure to halt its deadly offensive against Kurdish forces.

A US withdrawal from Syria's northern border this week has exposed Washington's Kurdish partners to a Turkish assault, creating conditions for a rapprochement with Damascus that both sides need badly, analysts said.

The main US partner in the years-long battle against the Islamic State group, Kurdish forces were left to fend for themselves after allied US forces pulled back on Monday.

Turkish forces have taken control of the Syrian border town of Ras al-Ain, which has been a focus of its operation against Kurdish militants, the defence ministry said Saturday. 

Pakistan has offered rare backing to Turkey in its offensive against Kurdish forces in Syria, with President Recep Tayyip Erdogan expected to visit Islamabad later this month.

Turkey denied on Saturday targeting a US base in northern Syria after the Pentagon said its troops had come under artillery fire. 

In the vanguard of Syrian revolt before being ripped apart by division and indiscipline, the Free Syrian Army has returned as a key component of Turkey's offensive against Kurdish militants.

Facing a backlash for appearing to greenlight Turkey's assault against Kurdish forces in Syria, President Donald Trump on Friday dialed up pressure on America's NATO ally by threatening crippling sanctions.

The United States was moving to quash accusations of mixed messages and policy reversals over Turkey's offensive into northeastern Syria, which began after Trump ordered US troops to pull back from the border.

France's foreign ministry on Thursday summoned the Turkish ambassador to Paris over Ankara's air and ground offensive into Kurdish-controlled areas of northern Syria, a diplomatic source said.