"We are not in a position right now to collaborate on the military level, but our political leaders will engage and try to find common ground", Mattis said, but the USA and North Atlantic Treaty Organisation could only negotiate with Moscow "from a position of strength".
The SDF, seen by the Washington as the most effective anti-jihadist fighting force in Syria, is dominated by Kurdish fighters condemned by Ankara as "terrorists".
The immediate concern is the tactical situation in and around the northeastern Syrian town of Manbij, where various and opposing forces supported by the U.S., Russian Federation and Turkey have closed to "within hand grenade range of one another", according to Army Lt. Gen. Stephen Townsend, commander of Combined Joint Task Force-Operation Inherent Resolve.
The spokesman said the new USA presence is meant in part to "reassure that ISIS has been driven from Manbij", adding, "Manbij is liberated and there's not a need for further fighting there".
Some U.S. troops are stationed inside and in rural parts of western Manbij since last week to deter actors from attacking groups other than the Islamic State (IS), Pentagon spokesman Captain Jeff Davis said on Monday. Syrian troops are reportedly not in Manbij, itself, but are patrolling areas to the west and south.
Although Turkey has spent years trying to unseat the regime of Bashar Al Assad by funnelling arms to rebel groups, Ankara and its proxies have avoided combat with government forces since the Turkish military intervention last year, focusing on the pursuit of ISIL and the YPG.
The U.S. has moved 500 soldiers to the outskirts of Manbij, according to Ilnur Cevik, chief adviser to Erdogan.
He said the handover was carried out "with the goal of curbing Turkey's expansion and its occupation of Syrian territory, and to. prevent the shedding of civilian blood".
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"Our plan is very clear, but the negotiation process is going on", Kalin said, referring to a trilateral meeting between U.S., Russian and Turkish military chiefs this week. Hailed as a significant military victory over the militants, the capture of Manbij cut the road between their capital in Raqqa and the Turkish border and paved the way for the ongoing, four-month-old offensive led by Kurdish forces to isolate Raqqa from the surrounding countryside. "We can not do what we're doing without them", he said.
That followed an agreement with Russian Federation to pre-empt an attempt by Turkish-led forces to take Manbij, Reuters reported. "It's a place where multiple forces, frankly, have converged - all with the intent to drive out ISIS", acting State Department spokesman Mark Toner told reporters in Washington.
The meeting of the top brass came ahead of a visit by Turkish President Recep Tayyip Erdogan to Russian Federation.
"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.
In the center of the country, Syrian troops captured the Jazal oil field from the extremists after days of fighting, state TV said, quoting an unnamed military official. But the USA 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.
"Our operations in Syria will continue, and our priority right now is to limit any adverse effects our departure from Turkey may have on the innocent men, women and children who depend on our assistance", Mercy Corps said.
And as a Turkish official admitted to Reuters, the United States is committed to conducting Raqqa with the SDF, not Turkey.