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.

The head of a Turkish opposition news site was arrested Thursday, his employer said, as part of a crackdown on critics of the military operation launched in Syria a day earlier.

A bipartisan group of US senators said Wednesday they have agreed on far-reaching sanctions to be slapped on NATO ally Turkey if its forces do not withdraw from neighboring Syria.

US President Donald Trump has warned Turkey against going too far in Syria, just a day after giving Ankara an apparent green light to invade its southern neighbor and sparking panic among Washington's Kurdish allies.

The US pulled back 50 to 100 "special operators" from Syria's northern frontier, where they have served as a buffer preventing a long-planned assault by the Turkish military against Kurdish forces in Syria. 

Turkey's preparations for an offensive in northern Syria have been "completed", the defence ministry said Tuesday, after confusing signals from the US over whether it would allow an operation.

US President Donald Trump ordered the withdrawal of US troops from Turkey's border with Syria after a phone call with his counterpart Recep Tayyip Erdogan. 

Iran's top diplomat Mohammad Javad Zarif told his Turkish counterpart Mevlut Cavusoglu in a phone call that Tehran is opposed to military action in Syria, the foreign ministry said.

US troops in northern Syria will no longer be near the border with Turkey, nor will they support Ankara's "long-planned operation" into the country, the White House said on Sunday.

An official with the main Kurdish-led force in Syria said Sunday the ethnic minority is not represented on the committee formed earlier this month that will be in charge of drafting a new constitution.

The Syrian government does not trust the main Kurdish militia known as the People's Protection Units, or YPG, and its political wing because of its links to the U.S. Turkey, a main backer of the opposition, sees the YPG as part of a Kurdish insurgency in Turkey.