BEIRUT (Reuters) – U.S.-backed Syrian militias advanced further into Islamic State’s Syrian stronghold of Raqqa from the east on Monday, reaching the walls of the Old City, a war monitor and a militia spokesman said on Monday.

The Syrian Democratic Forces (SDF), a group of Kurdish and Arab militias supported by a U.S.-led coalition, began to attack Raqqa last Tuesday with the aim of taking it from Islamic State militants, after a months-long campaign to cut it off.

The Syrian Observatory for Human Rights monitor said the SDF took the al-Sanaa industrial neighborhood on Monday as part of their push into the eastern half of the city, and had reached the walls of the Old City neighborhood.

SDF media officer Ahmad Mohammed said the SDF had reached the walls but there were still fierce clashes in al-Sanaa and the district had not yet been totally secured.

The Old City, east of central Raqqa, is a neighborhood of modern housing bordered on two sides by fortified city walls built in the eighth century by the Abbasid Islamic Caliphate which at one point used Raqqa as its capital.

Residents said on Monday the Old City area was being shelled intensely.

The U.S.-led coalition estimates that Raqqa, which Islamic State seized from Syrian rebels in 2014 during their lightning advance in Syria and Iraq, is defended by 3,000 to 4,000 jihadists.

It has been a hub both for Islamic State’s military leaders and its bureaucrats, and has been used to plot attacks in countries around the world.

The SDF also advanced from north of the city on Monday, taking a sugar factory complex northeast of Raqqa. A video said to show SDF officers within the complex shows heavy damage to the factory.

Since the offensive began the SDF, supported by heavy coalition air strikes, have taken territory to the west, east and north of the city.

The fighting has caused large numbers of people to flee the city and surrounding areas.

(Reporting by Lisa Barrington; Editing by Angus MacSwan)