US-backed Syrian fighters broke into the western part of the Islamic State (IS) militant group's stronghold Raqa on Saturday, they and a monitor said, opening up a second front inside the northern Syrian city.
The Syrian Democratic Forces alliance of Kurdish and Arab fighters has spent months tightening the noose on IS-held Raqa and entered the city for the first time earlier this week from the east, backed by US-led coalition air strikes.
On Saturday, they pierced into Raqa from the west, according to the Syrian Observatory for Human Rights monitoring group.
"The SDF captured the western half of the Al-Sabahiya neighbourhood and are reinforcing their positions there," Observatory head Rami Abdel Rahman said.
The forces then advanced north into the adjacent district of Al-Romaniya and seized part of it in the evening, Abdel Rahman told AFP.
Earlier, a statement from SDF's Operation Wrath of the Euphrates said its fighters entered Al-Romaniya on Saturday and were locked "in fierce fighting inside the district".
Held by IS since 2014, Raqa emerged as a key hub for IS operations in Syria, neighbouring Iraq, and beyond.
The SDF, which was formed in 2015, launched its campaign to capture Raqa in November and chipped away at IS territory around the city's north, west, and east.
In addition to holding part of Al-Sabahiya and Al-Romaniya, the alliance also controls Raqa's eastern neighbourhood of Al-Meshleb.
But the force has struggled to advance from the city's north, where IS holds a military complex known as Division 17.
"IS has reinforced the northern approach to Raqa much more, thinking that's how the SDF would try to advance on the city," Abdel Rahman said.
"The western and eastern entrances to the city were much less fortified," he said.
The offensive has been backed by the US-led coalition with air strikes, special forces advisers, weapons, and equipment.
Coalition strikes on the city throughout the day on Saturday killed at least 13 civilians, according to the Observatory.
Abdel Rahman said the recent raids brought the civilian death toll in coalition raids to 47 since the battle for Raqa city was launched on June 6.
Reports of civilian casualties in the air campaign have swelled in recent weeks, but the coalition says it takes every measure to avoid hitting non-combattants.
An estimated 300,000 civilians were believed to have been living under IS rule in Raqa, including 80,000 displaced from other parts of Syria.
Thousands have fled in recent months, and the UN humanitarian office estimates about 160,000 people remain in the city.
On Saturday, displaced families could be seen arriving at a makeshift camp in Ain Issa, 50 kilometres (30 miles) north of Raqa.
Children sprayed each other with water from large red water tanks, trying to cool off under the beating summer sun.
Last month a camp administrator said more than 20,000 people had sought refuge in Ain Issa, and appealed for additional assistance.