A US-backed Arab-Kurdish alliance on Friday severed the Islamic State group's main supply route to Turkey after encircling a key jihadist-held town in northern Syria, a monitor said.

"The Syrian Democratic Forces cut off the last road from Manbij to the Turkish border on Friday morning," the Syrian Observatory for Human Rights said.

IS still controls territory along the Turkish border with secondary roads to the frontier but these are more dangerous and difficult to access, Observatory head Rami Abdel Rahman said.