US-backed force gains ground on IS in Syria's Raqa

US-backed fighters gained ground against the Islamic State group in the streets of Raqa on Wednesday, a day after their months-long offensive finally broke into the jihadists' Syrian bastion.

The Syrian Democratic Forces militia has spent seven months advancing on the city, with backing from the US-led coalition bombing IS in Syria and neighbouring Iraq.

Captured by the jihadists in 2014, Raqa became synonymous with IS atrocities including beheadings and public displays of bodies, and also emerged as a hub for planning attacks abroad.

On Tuesday, the SDF's Arab and Kurdish fighters finally broke into the eastern Al-Meshleb district of the city.

Early on Wednesday, they captured the neighbourhood and the Harqal citadel to the west of the city, the command of "Operation Wrath of the Euphrates" said.

The citadel sits on a hilltop roughly two kilometres (just over a mile) from the city limits.

Fighting was also raging in a military complex around two kilometres north of the city, the Syrian Observatory for Human Rights monitoring group said.

An activist with the Raqa is Being Slaughtered Silently group said people in the city described non-stop bombardment.

"The bombing has been going for two days and hasn't stopped for more than an hour, it's air strikes, artillery fire and sometimes rockets," Abu Mohamed told AFP.

He said shops were barely open and that there were cuts in electricity and water supplies.

The Britain-based Observatory said the US-led coalition had carried out heavy bombing raids to back the advance.

One of Tuesday's air strikes inside the city killed eight civilians, including three children, Observatory head Rami Abdel Rahman told AFP.

- 160,000 inside Raqa -