Air strikes by the US-led coalition killed at least 21 civilians in and around a stronghold of the Islamic State group in northern Syria on Monday, a monitor said.

At least 15 civilians were killed in raids in a northern district of Manbij while six others were killed in a village near the city, said the Syrian Observatory for Human Rights.

The jihadist bastion lies on their main supply route between Syria and Turkey in the northern province of Aleppo.

It has faced a US-backed offensive by Kurdish and Arab fighters since May 31 that has caused thousands of civilians to flee.

But tens of thousands of civilians are still trapped in Manbij, most of which is controlled by the jihadists.

On June 23, the Syrian Democratic Forces alliance of Kurdish and Arab fighters broke into western districts of Manbij, after blocking the road south of the city heading to IS's de facto capital of Raqa.

But their advance has been slowed in the past month because of landmines planted by the jihadists, which are also fighting back by launching suicide attacks against the SDF.

Founded in October, the Kurdish-dominated alliance has seized territory from IS across large parts of northern and northeastern Syria.

Capturing Manbij would be its most significant victory yet.

Syria's war has killed more than 280,000 people and displaced millions since it started in 2011 with the brutal repression of anti-government protests.

Source: AFP