Fighters from the Syrian Democratic Forces (SDF) have broken into the western part of ISIL’s stronghold Raqqa, opening a second front inside the northern Syrian city.

The SDF, a US-backed group of Kurdish and Arab militias, said on Saturday that they had captured the western district of al-Sabahia, just hours after after taking the eastern district of al-Mishlab.

The UK-based Syrian Observatory for Human Rights (SOHR) said the SDF had seized al-Mishlab neighbourhood and had taken half of al-Sabahia, adding that they were advancing into Raqqa’s northwestern district of al-Romania.

“The SDF captured the western half of the al-Sabahiya neighbourhood and are reinforcing their positions there,” SOHR Rami Abdel Rahman said.

“They then advanced north to the adjacent district of Al-Romaniya and are fighting IS there,” he told AFP, using a different acronym to refer to ISIL, or the Islamic State of Iraq and the Levant group.

A statement from the SDF’s Operation Wrath of the Euphrates said its fighters stormed al-Romaniya on Saturday and were locked “in fierce fighting inside the district”.

The SDF, led by the Kurdish YPG armed group, have been encircling Raqqa, ISIL’s self-proclaimed capital in Syria, since November.

Backed by US-led air power, the SDF began a major offensive on Tuesday to seize the city that ISIL captured from other opposition groups in 2014.

Between 3,000 and 4,000 ISIL fighters are thought to be holed up in Raqqa, where they continue to erect defences against the anticipated assault.

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The city has been a hub for ISIL’s military leaders and bureaucrats, and has also functioned as a base from which the group has plotted attacks in countries around the world.

But the battle for Raqqa has been marked by fears for the thousands of civilians still trapped in the city, particularly as the SDF begins what is expected to be an all-out battle for control of dense urban space.

Geert Cappelaere, UNICEF’s regional director, said in a statement on Saturday that an estimated 40,000 children were trapped inside the city and enduring extremely dangerous conditions.

He said the conflict had resulted in massive displacements in and around Raqqa, with up to 80,000 children now internally displaced in Syria and living in temporary shelters and camps.

His comments came amid renewed claims that the US-led coalition was using white phosphorus-loaded munitions on densely populated areas.

Videos allegedly filmed in Raqqa and posted on social media showed highly incendiary munitions exploding over homes and buildings.

International law permits the use of white phosphorus in order to cover troop movements, but the use of the munition in densely populated areas violates the requirement under international humanitarian law for all feasible precautions to be taken to avoid civilian injury and loss of life.

The Pentagon described white phosphorus as a “chemical weapon” when it was used by Saddam Hussein against the Kurds, but has accepted uses of the substance in the past.