"The Syrian Democratic Forces (SDF) cut off the last road from Manbij to the Turkish border today 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.
This week the SDF, backed by air strikes by a US-led coalition, cut the road north out of Manbij to the IS-held border town of Jarabulus, which the jihadists had used as a transit point for fighters, money and weapons.
"For the jihadists to reach the Turkish border from Raqa, they now have to take a route that is more dangerous because of regime troops nearby and Russian air strikes," Abdel Rahman said.
Russia launched air strikes in support of President Bashar al-Assad's regime in Syria in September.