The Pentagon spokesman's remarks indicated that the USA does not approve of the Turkish plan to drive the Kurdish militia from the town.
The YPG is the dominant party in a USA -backed alliance, the Syrian Democratic Forces, now controlling Manbij. With the U.S. -Russian presence outside the city, the Turks seem to be backing down.
Meanwhile, Turkish Prime Minister Binali Yildirim said Turkey, the United States and Russian Federation must coordinate to clear Syria of terrorist groups.
The Pentagon said on Monday that U.S. forces have also taken up positions on the outskirts of Manbij to try to keep a lid on tensions.
Analysts advised against a Turkish military move to capture Manbij, warning Turkey would find itself confronting all other actors in the Syrian theatre.
Last month, the US and Russian Federation issued a joint statement as co-chairs of the International Syria Support Group to advocate a cessation of hostilities that excluded ISIS and affiliates of al-Qaeda or other groups designated as terrorist organizations by the U.N. Security Council.
In January, the SDF was provided by the US with armored vehicles for the first time ahead of the planned Raqqa offensive.
Ankara, incensed that USA -led coalition forces opted to back the coalition of Arab and Kurdish paramilitaries - known as the Syrian Democratic Forces, or SDF - to battle Islamic State, launched the operation in August. Raqqa, the de facto IS capital, is now besieged by the SDF militia.
"It's a visible reminder, for anybody who's looking to start a fight, that the only fight that should be going on right now is with ISIS", Davis said, using the Trump administration's preferred acronym for the Islamic State group.
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American defence officials said decisions over the Raqqa offensive had not been discussed with Turkish commanders in Antalya.
The fighters "have a lot of worry, but they don't have any power", he said. But last-minute maneuvering by Russia pre-empted the Turkish gambit with the SDF reportedly ceding control of the villages to Russian and Syrian regime forces.
"There is a need for an effective coordination in the efforts to clear Syria of all terror groups because so many countries are involved there", he said.
"Our plan is very clear, but the negotiation process is going on", Ibrahim Kalin said, referring to a trilateral meeting between U.S., Russian Federation and Turkey military chiefs this week.
Turkish forces have already pushed deep into northern Syria to retake Islamic State-held territory along its borders.
Turkish President Recep Tayyip Erdogan, addressing a campaign rally in the Turkish capital Ankara, reiterated his readiness to confront the Kurdish forces.
"Turkey's biggest setback in Syria is the fact that it does not have any other ally than the FSA, which is not sufficiently strong", argued Uzgel, who taught at Ankara University for many years. Turkey and Syria support opposite sides in the Syrian civil war.
Photographs and videos posted on social media in recent days have shown convoys of US troops, including Stryker armored vehicles and Humvees, heading through the northern Syrian countryside trailing big Stars and Stripes flags. But the US views the Kurds as an essential element of the battle against the radical Sunni group that's waged a global campaign of terrorist attacks from its self-declared caliphate in Syria and Iraq.
And as a Turkish official admitted to Reuters, the United States is committed to conducting Raqqa with the SDF, not Turkey.