Clashes in al-Raqa are continuing Tuesday (June 13th) between the Syrian Democratic Forces (SDF) and "Islamic State of Iraq and Syria" (ISIS) extremists, as they try to repel SDF fighters' advance towards the walls of the Old City, AFP reported.

The alliance of Kurdish and Arab fighters entered al-Raqa a week ago, after months of fighting to encircle the northern city.

Since then, they have seized one neighbourhood in western al-Raqa 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 al-Raqa, said ISIS was putting up stiff resistance.

"There is fierce fighting against ISIS which is making heavy use of mines and snipers and sometimes car bombs," she said.

The Syrian Observatory for Human Rights also reported heavy ISIS attacks against SDF fighters in the area.

"The district is not yet completely secured because of the repeated jihadist attacks," the monitor said.

Observatory director Rami Abdel Rahman said the capture of al-Senaa would be the SDF's "most important advance in the battle for al-Raqa because it brings them to the centre of the city".

"The main battle for al-Raqa will take place in the city centre," he said.

He added that a large number of ISIS fighters were holed up in the Old City, where the group is also believed to have dug tunnels to facilitate their defence of the area.