President Recep Tayyip Erdogan said Turkey would begin the operation "within days" to target the Kurdish People's Protection Units (YPG), which Ankara considers a "terrorist offshoot" of the outlawed Kurdistan Workers' Party (PKK),

"Unilateral military action into northeast Syria by any party, particularly as US personnel may be present or in the vicinity, is of grave concern," Pentagon spokesman Commander Sean Robertson said in a statement.

"We would find any such actions 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.

American forces are with the SDF east of the Euphrates as well as in the flashpoint city of Manbij, which is west of the river.

"Coordination and consultation between the US and Turkey is the only approach to address issues of security concern in this area," Robertson said.

"We are committed to working closely and recently held a high level working group on Syria with our Turkish partners precisely to enhance our cooperation, coordination, and consultation."

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

"We should not and cannot allow (IS) to breathe at this critical point or we will jeopardize the significant gains we have made alongside our coalition partners and risk allowing (IS) to resurge," Robertson said.

The Pentagon on Tuesday said American observation posts in northern Syria, meant to prevent altercations between the Turkish army and the YPG, have been erected, despite Ankara's request to scrap the move.