US-backed fighters gained ground against the Islamic State group in the streets of Raqa on Wednesday, their command said, a day after breaking into the jihadist's Syrian bastion.

The Syrian Democratic Forces have spent months advancing on the northern city and finally thrust into the eastern neighbourhood of Al-Meshleb on Tuesday. 

Early on Wednesday, the Arab and Kurdish fighters 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.

The Syrian Observatory for Human Rights said there was also fighting inside the Division 17 military complex, around two kilometres north of Raqa, but the area had been heavily mined by IS.

The Britain-based monitoring group said the US-led coalition had carried out heavy bombing raids on the city overnight.

Captured by the jihadists in early 2014, Raqa became notorious as a hub for IS's operations in Syria, Iraq and beyond.

The city has been the scene of some of the group's worst atrocities, including gruesome executions, public displays of bodies and the trafficking of women.

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 to other parts of the province or to makeshift camps in territory newly captured by the SDF. 

Formed in 2015, the SDF is an alliance of Arab and Kurdish fighters that has been backed by the US-led coalition with air support, military advisers and weapons deliveries. 

Source; AFP