US-backed forces in Syria have entered the most heavily fortified area of Raqa, the de facto capital of the Islamic State group, in what a US official says is a "key milestone" in the war against the jihadist force.

Success in Raqa and major advances by US-backed forces in Mosul, a second IS stronghold in Iraq, represent a powerful double blow to the violent extremist group.

The US Central Command said in a statement dated Tuesday that coalition forces supported an advance by Syrian Democratic Forces fighters "into the most heavily fortified portion of Raqa by opening two small gaps in the Rafiqah Wall that surrounds the Old City."

The SDF faced heavy resistance, as the IS fighters used the wall as a combat position and planted mines and improvised explosive devices against advancing fighters.

"Conducting targeted strikes on two small portions of the wall allowed coalition and partner forces to breach the Old City at a locations of their choosing," the statement read.

This prevented IS from using "pre-positioned mines, IED and VBIEDs, protected SDF and civilian lives, and preserved the integrity of the greatest portion of the wall."

A 25-meter (80-foot) section of the wall was targeted, which "will help preserve the remainder of the overall 2,500-meter wall," it added.

- 'Key milestone' -

Brett McGurk, the US special presidential envoy for the coalition to defeat IS, said on Twitter that breaching the wall in Raqa was a "key milestone in campaign to liberate the city."

The US-backed fighters entered Raqa from the south for the first time on Sunday, crossing the Euphrates River to enter a new part of the Syrian city, a monitor said.

The SDF have spent months closing in on the IS bastion and entered the city's east and west for the first time last month.

According to the coalition, some 2,500 IS jihadists are defending the city.

IS overran Raqa in 2014, turning it into the de facto capital of its self-declared "caliphate."

The city was the scene of some of the group's worst atrocities, including public beheadings.

The United Nations warn that up to 100,000 civilians are still trapped in the city.

McGurk earlier tweeted: "#ISIS terrorists down to less than one square kilometer in #Mosul and totally surrounded in #Raqqa, #SDF advancing from four directions."

- Final stages of Mosul battle -

In Mosul, Iraqi forces face stiff fighting and a rising number of suicide attacks, including some by female bombers, as they enter the final stages of the battle.

More than eight months since the start of the operation to retake Mosul, IS fighters have gone from fully controlling the northern Iraqi city to holding a limited area on its western side.

Iraqi forces have been closing in on the Old City in west Mosul for months, but the terrain combined with a large civilian population has made for an extremely difficult fight.

IS overran large areas in Iraq north and west of Baghdad in 2014, but Iraqi forces backed by US-led coalition air strikes have since regained much of the territory they lost.

The Iraqi military believes that there are just a few hundred jihadists left in Mosul.

Source: AFP