The Syria Democratic Forces (SDF) will be aided by US-led coalition air strikes.
They have also warned civilians to steer clear of areas where IS militants are present.
The coalition of Kurdish and ethnic Arab militias has been gaining ground in areas to the north of the city.
The offensive was announced at a press conference in Ain Issa, about 50 kilometres (30 miles) away from Raqqa.
“The major battle to liberate Raqa and its surroundings has begun,” a spokesperson for the SDF said.
An official in Washington also confirmed the start of the operation, saying forces would first attempt “to isolate Raqqa, to set the stage for an eventual assault on the city itself to liberate it.”
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Image courtesy of Reuters