This came after more than 20,000 civilians were evacuated from the IS-held area.
SDF spokesman Mustafa Bali told AP: "The battle is very fierce. Those remaining inside are the most experienced who are defending their last stronghold. According to this you can imagine the ferocity and size of the fighting."
Since 2014, RAF aircrew based in Cyprus have been flying near daily missions over Iraq and Syria as part of a coalition of nations fighting IS.
Operation Shader began as a humanitarian aid mission but has grown into a complicated mission to try and defeat IS.
The UK has been the second biggest contributor to the coalition of nations fighting IS, only behind the United States.