Thousands trapped as SDF forces surround Manbij

A siege by US-backed Kurdish and Arab forces of the key jihadist-held town of Manbij in northern Syria left tens of thousands of civilians trapped yesterday.

Near Damascus, suicide bombings claimed by the Islamic State group outside a Shia shrine killed at least 20 people, the latest deadly strike on the revered site.

The Syrian Democratic Forces alliance supported by US air strikes encircled Manbij on Friday, severing the jihadist group's main supply line between Turkey and its de facto Syrian capital of Raqa.

Manbij lies at the heart of the last stretch of IS-controlled territory along Turkey's border, and it was a key point on the jihadists' supply line.

The Syrian Observatory for Human Rights said warplanes from a US-led coalition have carried out heavy bombing raids on and around the town held by IS since 2014.

"Tens of thousands of civilians still there can't leave as all the routes out of town are cut," the Britain-based monitor's head Rami Abdel Rahman said.

He said at least 159 IS fighters and 22 SDF troops had been killed as well as 37 civilians, most of the last group in coalition air raids, since the alliance offensive against Manbij began on May 31.

Dakish Fatimi, a Kurdish Red Crescent official, said his team had treated dozens of civilians wounded by landmines planted by retreating IS fighters.

South of Manbij, pro-regime fighters backed by Russian air strikes yesterday were 15 kilometres from the jihadist bastion of Tabqa's military airport.

Near the capital yesterday, bombings outside a Shia shrine killed at least 20 people including 13 civilians, the Observatory said. IS claimed the attacks saying they were carried out by three suicide bombers.

IS bombings near the shrine in February killed 134 people, mostly civilians. In January, another IS attack killed 70 people.

Lebanese Shia militant group Hezbollah cited the threat to Sayyida Zeinab as a main reason for intervening in Syria's war. The shrine contains the grave of Zeinab, a venerated granddaughter of the Prophet Mohammed.