The United States has voiced "grave concern" over a military operation planned by Turkey in northeastern Syria.

"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," said Pentagon Spokesman Cmdr. Sean Robertson on Dec. 12. "We would find any such actions unacceptable."

He said, "coordination and consultation between the U.S. and Turkey is the only approach to address issues of security concern in this area."

The U.S. military is committed to working closely with the Turkish military to boost cooperation and coordination, said Robertson, adding "uncoordinated military operations" will undermine that shared interest. 

"We have solemn obligations to one another's security. We are fully committed to Turkey's border security," he said.

President Recep Tayyip Erdoğan had announced earlier Dec. 12 that Turkey will start a military operation east of the Euphrates river in northern Syria in a "few days."

"We will start the operation in east of the Euphrates in a few days to save it from the separatist terrorist organization," Erdoğan added, referring to the YPG. "Turkey's target is never the U.S. soldiers, but rather the members of the terror group."

Ankara deems the YPG as an offshoot of the PKK, which is listed a terrorist organization by Turkey, the U.S. and the European Union.

Turkey is also skeptical about a U.S. plan to train around 40,000 locals in northeastern Syria.

Robertson said Dec. 12 that the U.S.-led coalition is working closely with the YPG/PKK-led SDF group, which he said is "in the midst of offensive operations against ISIS in the Middle Euphrates River Valley," using another name for ISIL.

"The SDF remains a committed partner against ISIS and we remain committed to working with them to ensure ISIS's enduring defeat," he said.

"There is no Daesh threat in Syria any longer," Erdoğan said Dec. 12, using another name for ISIL, and accusing the U.S. of "delaying tactics" regarding its promise to clear the northeastern Syrian town of Manbij from YPG members.