130 IS fighters killed in battle for Syria's Manbij

More than 130 Islamic State group fighters have been killed in a US-backed offensive on the key militant-held city of Manbij in northern Syria, a monitoring group said on Thursday.

US-led coalition airstrikes supporting the assault by Kurdish and Arab fighters, launched on 31 May, have also left 30 civilians dead, said the Syrian Observatory for Human Rights, which relies on a network of activists and medical sources inside Syria.

"Most of the Daesh fighters were killed in air raids by the international (US-led) coalition," Abdel Rahman told AFP, using an Arabic acronym for the group.

He said dozens of bodies of IS fighters had been found on Thursday morning in small villages east of Manbij.

Coalition air raids supporting the assault also killed at least 30 civilians, including 11 children, the Observatory said.

They are among a total of 447 civilians killed in coalition raids since they began in Syria in September 2014, according to the monitor's tally.

The Observatory says it determines whether strikes are carried out by Syrian, Russian or US-led coalition aircraft based on their locations, flight patterns and the types of planes and munitions involved.

The Syrian Democratic Forces (SDF) have been pushing west from the Euphrates River and have nearly encircled Manbij, a key point along IS's main supply line from the Turkish border to its eastern Syrian stronghold of Raqa.

The SDF alliance has surrounded the city from the north, east and south. Early on Thursday, its fighters were advancing towards the main road leading west out of Manbij, according to Observatory head Rami Abdel Rahman.

A statement on Thursday by the SDF's Manbij operations centre said its fighters were now close enough to target IS positions inside the city.

Washington hopes the operation will choke off Islamic State's last major link to the outside world, as the militants have used the border for years to receive supplies and manpower, and more recently to send back fighters for attacks in Europe.

If Manbij is captured, it will be the extremists' biggest defeat in Syria since government forces captured the central historic town of Palmyra in March.

A spokesman for the US defence department said on Wednesday that the final assault on Manbij could take place within days.