U.S. troops withdrew on Sunday from another town in Syria as Turkish-backed forces pushed deeper inside Syrian territory, seizing positions along a major highway that serves as the U.S. military’s main supply route and cutting off access to U.S. troops further west, according to a U.S. official. ...

Ain Issa, 20 miles south of the Turkish border, served as the headquarters of the Kurdish led administration in northeast Syria and owes its significance to its position beside the important M4 highway, which runs across northeastern Syria from the Iraqi border. It is the main supply route in and out of Syria for the 1,000 U.S. troops deployed there, as well as for much of the limited aid that reaches northeastern Syria...

Turkish backed rebels have now set up checkpoints on the highway near Ain Issa, cutting off the U.S. troops in bases to the west, in Manbij and Kobane. Those troops came under Turkish artillery fire Friday night in what some U.S. soldiers suspect was a deliberate attempt to drive them away from the bulk of the U.S. forces further east, Kurdish and U.S. officials said. ...

But with their supply lines severed, their Kurdish allies focused on fighting Turkey and not the Islamic State and the region in disarray, the entire U.S. presence in northeast Syria is starting to appear increasingly unsustainable, U.S. officials and analysts say.