WASHINGTON • US-backed forces in Syria have breached the wall surrounding the Old City of Raqqa in Syria as they try to retake the city from the Islamic State in Iraq and Syria (ISIS) militant group.

Air strikes by the US-led coalition battling ISIS punched two holes in the mediaeval wall, allowing fighters of the Syrian Democratic Forces (SDF) to penetrate the militants' defences, Washington and the SDF said yesterday. The SDF is an Arab-Kurdish alliance that was formed in 2015.

The US Central Command said the two breaches enabled fighters to evade the extensive defences the militants have put up over the past three years.

ISIS has "used this archaeological wall to launch attacks, and planted bombs and mines in its gates to hinder the advance of SDF forces", it said on Twitter.

Raqqa became infamous as the scene of some of the group's worst atrocities, including public beheadings, and is thought to have been a hub for planning attacks overseas.

The United Nations has raised concerns for tens of thousands of civilians trapped in Raqqa, where it says the militants are using many as human shields.

The landmark advance in Raqqa comes as the militants also face likely defeat within days, 375km to the east in Iraq's second city Mosul - the other pivot of the "caliphate" ISIS declared in 2014.

But Iraqi forces are facing increasingly difficult fighting there, with a rising number of suicide attacks, including some by female bombers, in the final stages of the battle, commanders said.

Iraqi forces were moving in from all sides on the last ISIS-controlled neighbourhoods of the Old City yesterday, commanders said, adding that they expected victory in as few as two days.

While coalition estimates put 2,500 ISIS militants as defending Raqqa, Iraqi commanders estimate only 200 or so diehard ISIS fighters, most of them foreign, are holed up in Mosul.

AGENCE FRANCE-PRESSE