SDF forces score 'key victory' against Islamic State in battle to recapture Syria's Raqqa

The US Central Command (Centcom) said Tuesday that coalition forces and SDF fighters advanced "into the most heavily fortified portion of Raqqa by opening two small gaps in the Rafiqah Wall that surrounds the Old City."

The SDF – made up of a coalition of Arab and Kurdish militiamen – faced heavy resistance from IS fighters, who had used the wall as a combat position and loaded it with mines and IED's.

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

After major advances in the Iraqi city of Mosul by US-backed Iraqi armed forces, success in Raqqa would represent a powerful double blow to the extremist group.

Islamic State on brink of collapse

The US coalition says around 2,500 IS fighters are defending the city, which IS captured in 2014 and turned into the de-facto capital of the self-declared "caliphate."

After IS took control, the city became a site for some of the group's worst atrocities, including public beheadings, floggings, and mutilated bodies left on public display.

Breaching the Old City wall is a "key milestone in campaign to liberate the city," Brett McGurk, the US special presidential envoy for the coalition, said on Twitter.

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

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."

In Mosul, Iraqi forces face stiff fighting and suicide attacks as they enter the final stages of battle.

Iraqi forces launched the operation to retake the Old City in mid-June and after a dawn push last Thursday, they retook the area around the Grand al-Nuri Mosque, which the militants had blown up just a few days earlier.

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

"Their fictitious state has fallen," Iraqi military spokesman, Brigadier General Yahya Rasool, said after Iraqi forces recaptured the iconic mosque.