US-backed Syrian fighters said they had seized a second district of Raqa on Sunday and launched a renewed assault on a base north of the city, as they pursued an offensive against the Islamic State (IS) militant group.

The Syrian Democratic Forces (SDF), who broke into Raqa after announcing the start of a final assault on the city last week, said their fighters "liberated the neighbourhood of Al-Romaniya on the western front of Raqa, after two days of continued clashes."

It was the first time the SDF was reported to have taken a western district of IS-held Raqa, which its fighters are bearing down on from the east, west and north. The SDF previously seized control of the district of Al-Meshleb in the east.

There was less progress though on the northern front of the battle, where the SDF has struggled to capture the Division 17 military base and an adjacent sugar factory, used by IS to defend approaches into the city.

SDF fighters were battling on Sunday to dislodge IS from the base, with backing from the US-led coalition bombing IS, according to the Syrian Observatory for Human Rights.

"Blasts could be heard throughout the night because of the exchange of fire between the two sides," the Britain-based monitoring group said.

Observatory head Rami Abdel Rahman said IS had "heavily fortified" the base in anticipation of a ferocious SDF assault on it.

Originally a Syrian army base, Division 17 was seized by IS in 2014 as it took control of swathes of the wider Raqa province.

After its capture by IS the same year, Raqa city emerged as a key hub for IS's operations in Syria, neighbouring Iraq, and beyond.

The SDF -- an Arab-Kurdish alliance formed in 2015 -- spent seven months tightening the noose on Raqa city before finally entering it this week.

After seizing Al-Meshleb on Wednesday, SDF forces were using it as a launching pad for new operations, according to the Observatory.

Al-Meshleb is one of the more built-up residential neighbourhoods in the city's east, while most other districts nearby are made up of markets and small shops.

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.

Reports of civilian casualties among those still living inside have swelled in recent weeks.

The Observatory said Sunday that coalition air strikes the previous day killed 24 civilians inside the city, up from an earlier toll of 13 people.

Abdel Rahman said the increased toll brought civilian deaths in coalition raids to a total of 58 since the battle for Raqa city was launched on June 6.

To back the assault on Raqa, the US-led coalition has provided the SDF with air cover, special forces advisers, weapons, and equipment.

The alliance first began bombing IS positions in Iraq in August 2014, and expanded its operations to Syria the following month.

In addition to heavy raids on Raqa, the coalition also pounded the IS-held town of Al-Mayadeen in eastern Syria on Sunday, according to the Observatory.

"Many of IS's second-tier leaders fled to Al-Mayadeen when the offensive for Raqa started months ago," said Abdel Rahman.

More than 320,000 people have been killed since Syria's conflict erupted in March 2011 with demonstrations against President Bashar al-Assad.

It has since turned into complex, multi-front conflict pitting IS, rebel groups, government forces, and Kurdish fighters against each other.