US-backed forces battling to oust the Islamic State group from its Syrian bastion Raqa have advanced in the city's south, seizing a new neighbourhood, a spokesman and monitor said Tuesday.

The Kurdish-Arab Syrian Democratic Forces militia (SDF) began a campaign to capture Raqa from IS last year, slowly encircling the city before breaking into it for the first time in June.

Backed by US-led coalition air strikes, the alliance now controls more than 50 percent of the city, according to the Syrian Observatory for Human Rights, a Britain-based monitor.

"Overnight, the SDF advanced in the south of the city, after taking control of the Nazlet Shahada neighbourhood," said Observatory director Rami Abdel Rahman.

He said SDF fighters also controlled large parts of the adjacent neighbourhood of Hisham Bin Abdel Malik, after advancing in the south from both the eastern and western fronts.

"Daesh (IS) effectively no longer has a presence in the southern neighbourhoods of Raqa, after SDF forces coming from the eastern front met with those advancing from the western front," he added.

Nuri Mahmud, a spokesman for the Kurdish YPG militia that dominates the SDF, confirmed to AFP that the SDF was advancing in the south.

"Daesh has been almost completely eliminated from the Nazlet Shahada and Hisham Bin Abdel Malik neighbourhoods," he added.

Writing on Twitter, the US envoy to the international coalition against IS praised the SDF's advances.

"Significant progress in Raqa last 24 hours, with key territory seized by SDF forces & 100s of civilians freed," Brett McGurk wrote.

Abdel Rahman said the fighting was now centred around the area south of the city centre and on the outskirts of the district of Hisham Bin Abdel Malik.

"The SDF is a few hundred metres (yards) from Daesh's main headquarters in Clock Square, which is where Daesh carried out executions," he said.

He added that SDF fighters were also on the outskirts of Al-Thakana neighbourhood, one of the city's most densely-populated.

SDF fighters have faced fierce resistance since they entered Raqa in early June, and the fighting has displaced thousands of civilians.

Mahmud said IS was "exploiting civilians and using mines, car bombs, drones, tunnels and suicide bombers to prolong its presence inside Raqa".

The United Nations estimates between 20,000 to 50,000 civilians may still be in the city, though other estimates put the numbers lower.

Aid groups warned Monday that food access had reached a "critical turning point" with markets shuttered and residents dependent on dwindling stockpiles.