The United States has increased its forces near the northern Syrian town of Manbij as concerns have grown that fighting could erupt among the complex array of militias and Syrian and Turkish troops who are operating on the crowded battlefield near there.

Photographs of American troops in Stryker vehicles and armored Humvees flying American flags circulated on Saturday on social media. The American-led command that is fighting the Islamic State, a militant group also known as ISIS or ISIL, acknowledged the buildup around the town, but did not provide details…

A combustible mix of forces are crowding the battlefield in northern Syria, including Turkish-backed Syrian militias, Syrian government forces, and Kurdish and Arab fighters trained by the United States.

Lt. Gen. Stephen Townsend, who commands the American-led task force that is fighting the Islamic State in Iraq and Syria, told reporters on Wednesday that Syrian government troops had advanced near Manbij to the point where they were within “rifle range” of Syrian Arab fighters backed by American forces.