US-backed forces in Syria have managed to enter a heavily fortified area in the city of Raqqa, a stronghold of the Isis terror group, in what has been deemed as "a key milestone" in the fight against the militants.

Raqqa has witnessed intensified fighting in recent weeks, after the Syrian Democratic Forces (SDF) – an alliance of Kurdish and Arab militias supported by a US-led coalition – started an offensive to recapture Raqqa from Isis. The group captured the city in 2014, turning it into the capital of its self-declared "Caliphate".

Coalition forces supported an advance by SDF "into the most heavily fortified portion of Raqqa by opening two small gaps in the Rafiqah Wall that surrounds the Old City," the US Central Command said in a statement on Tuesday (4 June).

"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 statement continued.

Isis fighters used the wall as a combat position and planted explosives in the surrounding areas to prevent forces from advancing.

Brett McGurk, the US special presidential envoy for the coalition to defeat Isis, hailed the breaching of the wall as a "key milestone in campaign to liberate the city."

Since the offensive began on 6 June, the SDF has recaptured territories to the west, east and north of the city. According to the coalition, around 2,500 Isis fighters are holed up in Raqqa.

The UN has warned that US air strikes conducted in support of the offensive are endangering a "staggering number" of civilians. The organisation estimated there are at least 100,000 people still "effectively trapped" in the city.

Isis used to control large swathes of territory in Syria and Iraq but it is progressively losing territories following joint offensives in both countries.

The battle for Mosul

Isis has lost most of the controlled areas in Mosul, Iraq's second largest city, which the group seized in 2014.

The Iraqi army, the Kurdistan Regional government and a US-led coalition are involved in what is known as the 'Battle for Mosul', a joint military offensive that began in October 2016.

The army recaptured eastern Mosul in January and is now fighting to retake control of the western part of the city. Troops are now advancing in the last pocket of territory held by the militants in Mosul's Old City neighbourhood, prompting the militants to destroy the al-Nuri Mosque in June.

The mosque was symbolic as it was used by Isis leader Abu Bakr al-Baghdadi to deliver his declaration of the establishment of the caliphate. Isis also used it as a base to become an unprecedented global terror threat.

It is believed Isis now controls just one square kilometre of the warren of alleyways in the city centre and the army estimates it can retake full control of the city in a matter of days. However, several previous timetables for the final defeat of the Isis hold-outs have been missed.

Iraqis prepared to celebrate the expected victory over Isis on Sunday (2 July), with troops tying white banners and Iraqi flags to lamp posts and damaged buildings, Reuters reported. The government said it was planning a week-long celebration.