Earlier, Ankara announced that the Turkish military would launch an operation against Kurdish forces.
"Unilateral military action into northeast Syria by any party, particularly as US personnel may be present or in the vicinity, is of grave concern," Robertson said on Wednesday when asked about Turkey’s announcement. "We would find any such actions unacceptable… coordination and consultation between the US and Turkey is the only approach to address issues of security concern in this area."
Uncoordinated military operations will undermine the shared US-Turkish interests in Syria, Robertson said. As a NATO ally and key partner in the Global Coalition against Daesh terrorist group*, both countries have solemn obligations to each other's security, he added.
The United States remains committed to Turkey's border security, he said.
US-Turkish relations have suffered a setback amid Ankara's concerns over US support for the Kurdish People's Protection Units (YPG). Ankara has also repeatedly accused Washington of failing to fulfil its promises regarding the withdrawal of the YPG from Syria’s Manbij.
Ankara regards YPG as an affiliate of the Kurdistan Workers' Party (PKK), outlawed in Turkey.
*Daesh (also known as ISIS/ISIL/IS) is a terrorist organization banned in Russia