IS battles to keep US-backed Syria force from Raqqa's Old City
The Kurdish and Arab fighters of the Syrian Democratic Forces (SDF) entered Raqqa city a week ago, after months of fighting to encircle the northern city that has become a militants' bastion.
Since then, they have seized one neighbourhood in western Raqqa and another in the east, where they are now battling to secure control of the al-Senaa district that leads to the Old City.
Jihan Sheikh Ahmed, spokeswoman for the SDF campaign for Raqqa, said the militant group was putting up stiff resistance.
"There is fierce fighting against Daesh [IS] which is making heavy use of mines and snipers and sometimes car bombs," she told AFP.
The Syrian Observatory for Human Rights monitor also reported heavy IS attacks against SDF fighters in the area.
"The district is not yet completely secured because of the repeated jihadist attacks," the UK-based group said.
Observatory Director Rami Abdel Rahman said the capture of al-Senaa would be the SDF's "most important advance in the battle for Raqqa because it brings them to the centre of the city".
"The main battle for Raqqa will take place in the city centre," he said.
He added that a large number of IS fighters were holed up in the Old City, where the jihadi group is also believed to have dug tunnels to facilitate their defence of the area.
Since entering Raqqa on 6 June, the SDF has captured the eastern neighbourhood of al-Meshleb, as well as al-Rumaniya in the city's west.
It is now battling to push from al-Rumaniya into neighbouring Hatin district.
Agencies contributed to this report.