US-backed Syrian forces cut IS supply route in march on Manbij

US-backed Kurdish and Arab fighters advanced on Thursday into the Islamic State (IS) group's bastion of Manbij in northern Syria, sparking fierce street fighting as they pushed to take the city.

Backed by US-led coalition air strikes, fighters with the Syrian Democratic Forces (SDF) alliance entered Manbij from the south, a monitoring group said.

The advance marked a breakthrough in the battle for Manbij, a key link in the supply route between the Turkish border and IS's de facto Syrian capital of Raqqa.

The loss of the city would deal another blow to IS after several recent battlefield defeats, including the re-taking of Fallujah by Iraqi forces earlier this month.

On the humanitarian front, the UN said it would begin flying desperately needed aid from Damascus to the northeastern city of Qamishli, which has been inaccessible by road for more than two years.

The Syrian Observatory for Human Rights, a Britain-based monitor, said SDF forces were able to break through IS defences in Manbij a few hours after taking control of a village on the city's southwestern outskirts.

Director Rami Abdel Rahman confirmed that clashes and air strikes had taken place on Thursday around Manbij.

An SDF commander at the front told AFP that IS militants used car bombs and explosives to try to slow the assault.

"Our forces, in coordination with the coalition, are determined to advance inside the city and eliminate all Daesh fighters," he said, using an Arabic acronym for IS.

Abdel Rahman said tens of thousands of civilians are trapped inside Manjib, though some 8,000 had fled since the start of the SDF offensive on 31 May.

He said six civilians including a child were killed on Thursday by a mine as they tried to flee the city, which had a population of about 120,000 before the start of Syria's civil war in 2011.

The SDF encircled the city on 10 June, but its advance slowed as IS fought back, including with almost daily suicide bombings.

At least 63 SDF fighters and 458 IS militants have been killed in the offensive, according to the Observatory.

The militants have held Manbij since 2014, the year IS seized control of large parts of Syria and neighbouring Iraq and declared its "caliphate".

The US-led coalition of Western and Arab states launched air raids against IS in both countries the same year, and in recent months has stepped up support for ground forces like the SDF.

A statement from US Central Command said the coalition had carried out 73 strikes in the Manbij area last week and a total of 233 since the assault began.

The coalition said on Thursday that the SDF has yet to penetrate the centre of Manbij.

British Major General Doug Chalmers told reporters that SDF forces were in "the outer element of the city rather than the city proper".

Formed in October 2015, the 25,000-strong SDF is dominated by the powerful Kurdish People's Protection Units (YPG), but includes an Arab contingent of about 5,000 fighters.

Five million besieged

Five million Syrians now live in besieged or difficult-to-reach areas, nearly a million more than previously estimated, UN emergency relief coordinator Stephen O'Brien said on Thursday.

The UN representative attributed the spike to heightened insecurity in parts of Aleppo, Raqqa and Hasakeh that has constrained access to those regions.

Since January, UN aid convoys have reached 334,150 of an estimated 590,000 besieged Syrians - the majority entrapped by government forces.

O'Brien said those deliveries marked progress but were only "a trickle, compared to the level of protection concerns, needs and suffering in besieged and hard-to-reach areas". 

It is "vital that the stuttering momentum sustained in humanitarian access over the past few months continues and improves, and is significantly expanded into the second half of this year," O'Brien said.

Syria's conflict began five years ago with the brutal repression of anti-government demonstrations. It has killed more than 280,000 people and displaced millions.