Syrian rebels vow to back Turkish operation against YPG as US urges caution

Thousands of Syrian rebels say they are ready join a Turkish military offensive against US-backed Kurdish forces in Syria, as the US cautioned against Ankara's "unacceptable" operation.

Turkish President Recep Tayyip Erdogan said on Wednesday that Turkey would launch a military operation within a few days to target the Syrian Kurdish militia known as the People's Protection Units, or YPG - which Turkey views as a terrorist group linked to the insurgency within its borders.

A spokesman for the National Army, a Turkish-backed rebel force in northwest Syria, said up to 15,000 Syrian rebels are ready to join the offensive.

"The battle will be launched simultaneously from several fronts," Major Youssef Hamoud told Reuters.

"It will be in Manbij and Tel Abyad and Ras al-Ayn," he said, referring to towns about 200 km apart near Syria's northern border.

The US said on Wednesday that any unilateral military action by Turkey would be "unacceptable".

American forces have worked closely with the YPG under the Kurdish-led Syrian Democratic Forces (SDF) alliance, which has played a key role in the war against the Islamic State (IS) extremist group.

The Pentagon has repeatedly warned that any fighting between the Turks and the SDF is a dangerous distraction from the core US mission in Syria of fighting IS.

Pentagon spokesman Commander Sean Robertson said any unilateral military action in northeast Syria would be a "grave concern", as it could potentially jeopardise US troops working with the SDF in the region. 

"We would find any such actions unacceptable," he said in a statement.

Turkish troops have already driven back Syrian Kurdish forces in two cross-border operations west of the Euphrates, in 2016 and earlier this year.