WASHINGTON, Dec. 12 (CGTN) -- Any unilateral military action in northern Syria would be "unacceptable," the Pentagon warned Wednesday after Turkey said it would soon launch an operation against a U.S.-backed Kurdish militia.

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 U.S. 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 ISIL 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 U.S. 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 U.S. mission in Syria of fighting ISIL.

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

Turkey to launch operation 'in a few days'

Ankara and Washington have long been at odds over Syria. Turkey has already intervened to sweep YPG fighters from territory west of the Euphrates in military campaigns over the past two years, but up until now, it had not gone east of the river – partly to avoid direct confrontation with U.S. forces.

But Erdogan's patience with Washington over Syria – specifically a deal to clear the YPG from the town of Manbij, just west of the Euphrates – seems to have worn thin.

"We will start the operation to clear the east of the Euphrates from separatist terrorists in a few days. Our target is never U.S. soldiers," Erdogan made the remarks during his speech at a defense industry summit in Ankara. "This step will allow for the path to a political solution to be opened and for healthier cooperation."

Turkey has repeatedly voiced frustration about what it says are delays in the implementation of the Manbij deal, saying last month that the agreement should be fully carried out by the end of this year.

Turkish and U.S. troops began joint patrols near Manbij last month, but that cooperation has also been complicated as Turkey has shelled Kurdish fighters to the east of the Euphrates.