US-backed force has seized a quarter of Raqqa from IS: monitor

US-backed fighters seized a quarter of Syria's Raqqa from the Islamic State group, a monitor said on Monday, less than three weeks after they first launched the offensive to recapture the northern city. 

Arab and Kurdish militiamen from the Syrian Democratic Forces smashed into the militants' main Syrian bastion on June 6 after a months-long drive to encircle it. 

"Since the offensive began, the SDF have captured around 25 percent of the city's built-up neighbourhoods," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP on Monday.

Backed by US-led coalition airstrikes, the SDF has fully seized the south eastern districts of al-Meshleb and al-Senaa, as well as al-Rumaniya and Sabahiya in the west, he said.

From those neighbourhoods, they were bearing down on Raqqa's Old City in a pincer movement on Monday, with fighting raging in the western al-Qadisiya district and parts of the city's east. 

SDF fighters also hold part of Division 17 - a former Syrian army base - and an adjacent sugar factory on the northern edges of the city.

"They want to cut off the city's northern part, including the Division 17 base, so that there's more pressure on IS in the city centre," Abdel Rahman said.

The battle for Raqqa is the SDF's flagship offensive, with heavy backing from coalition airstrikes, advisers, weapons and equipment. 

But on Friday, Syrian Observatory for Human Rights said the period between May 23 and June 23 saw the highest civilian death toll in coalition raids for a single month since they began on September 23, 2014.

Rahman said 260 civilians, including 82 children, were killed in the largely Islamic State group-held province of Deir az-Zour, and another 250 civilians, including 53 children, were killed in Raqqa.

He told AFP that the new deaths brought the overall civilian toll from the coalition's campaign to 1,953, including 456 children and 333 women.

IS overran Raqqa in 2014, transforming it into the de facto Syrian capital of its self-declared "caliphate".

It became infamous as the scene of some of the group's worst atrocities, including public beheadings, and is thought to have been a hub for planning attacks overseas.