WATCH: Anti-IS forces advance on Manbij as civilians try to flee

Tens of thousands of civilians are trapped inside the northern Syrian city of Manbij after US-backed fighters surrounded the Islamic State (IS) group stronghold, a monitor said on Saturday.

The Syrian Democratic Forces (SDF) encircled Manbij on Friday, severing IS's principal supply route between Turkey and its de facto Syrian capital, Raqqa.

The Syrian Observatory for Human Rights, a British-based monitor, said warplanes from a US-led coalition were conducting heavy bombing raids on Manbij.

"Tens of thousands of civilians who are still there can't leave as all the routes out of the city are cut," Observatory head Rami Abdel Rahman told AFP.

The US envoy to the anti-IS coalition backing the SDF, Brett McGurk, said the surrounding of Manbij had severed an important route for IS militants.

"ISIL terrorists now completely surrounded with no way out," he wrote on Twitter, using another name for IS. 

The city has been held by IS since 2014 and the Observatory said that thousands of inhabitants had recently fled Manbij as air strikes intensified.

"Bakeries in the town haven't been open since Friday and food is beginning to become rare," Abdel Rahman said. 

He said that at least 159 IS militants, 22 SDF troops and 37 civilians had been killed, mainly by bombing raids, since the alliance launched its Manbij offensive on 31 May. 

Last week, Syrian rebels pushed back IS militants from two strategic villages near the Turkish border, following weeks of defeats in the key pocket and alleviating fears that the militants would cut off supply lines.

IS had captured several villages between the rebel-held northern towns of Marea and Azaz on 27 May and last week launched a fresh drive to oust Syrian opposition forces from their last stronghold in the Azaz area.

The assault forced thousands of people to flee and sparked concerns from aid groups that 100,000 civilians could find themselves trapped, while tens of thousands of displaced Syrians who have been sheltering in makeshift camps next to the Turkish border would also come under fire.

Syria's civil war began with the brutal repression of anti-government demonstrations in 2011 and has now killed more than 280,000 people and displaced millions.