The Syrian Democratic Forces (SDF), a group of Kurdish and Arab militias supported by a US-led coalition, began to attack Raqqa on Tuesday after a months-long campaign to cut it off.
The US-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.