US-backed forces cut off all routes in and out of the Islamic State (IS) group-held city of Manbij near the Syrian-Turkish border on Friday, the Syrian Observatory for Human Rights monitoring group said.
The Syria Democratic Forces, supported by US-led air strikes and US special forces, seized control of the final road leading into Manbij and have encircled the city, the British-based Observatory said.
The offensive aimed at flushing IS group out of territory it controls along the border, its only major route to the outside world, began last week.
The SDF, which includes Syrian Kurdish and Arab fighters, had by Thursday advanced to within firing distance of the last main highway into Manbij.
The Observatory said the SDF effectively took control of the final road from Manbij. "There's no road left... they're all cut," its director Rami Abdulrahman said.
There was no immediate comment from the SDF.
The Observatory said nearly 160 Islamic State (IS) group fighters have died in battles with the SDF around Manbij. More than 20 SDF fighters have been killed, it said.