The Syrian Democratic Forces (SDF) gained ground against the "Islamic State of Iraq and Syria" (ISIS) in the streets of al-Raqa on Wednesday (June 7th), AFP reported.

The Arab-Kurd opposition alliance has spent seven months advancing on the city, with backing from the international coalition, and finally broke into the eastern Al-Meshleb district of the city on Tuesday.

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

The citadel sits on a hilltop roughly two kilometres from the city limits.

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

The Observatory said the coalition had carried out heavy bombing raids to back the SDF advance.

An estimated 300,000 civilians were believed to have been living under ISIS rule in al-Raqa, including 80,000 displaced from other parts of Syria.

But thousands have fled in recent months, with the UN humanitarian office saying on Tuesday that it estimated about 160,000 people remained in the city.

Its regional spokesman David Swanson said as many as 100,000 people "may stay trapped" inside during the assault.

The International Rescue Committee said it was "deeply concerned for the safety of civilians in al-Raqa" after a drop in numbers fleeing the city in the past week.

That decrease could indicate that ISIS intends to use the remaining civilians "as human shields", the aid group warned.