The number of US troops in Syria remains roughly stable at just under 1,000 three weeks after President Donald Trump announced their withdrawal, a US official said Monday.

The withdrawal of American troops from Syria's northern border opened the way for Turkey's military incursion against Kurdish forces in the country.

An air strike Saturday by key Damascus ally Russia killed six civilians including a child in the embattled opposition bastion of Idlib in northwestern Syria, a war monitor.

US forces patrolled part of Syria's border with Turkey on Thursday in the first such move since Washington withdrew troops from the area earlier this month, an AFP correspondent reported.

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.