A US-backed Kurdish and Arab force says it has begun an operation to capture Raqqa, so-called Islamic State’s “capital” in Syria.
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”.
Another SDF spokesperson told AFP that heavy resistance is expected. “IS will defend its bastion, knowing that the loss of Raqqa will mean it is finished in Syria,” he said.
The operation comes as US-backed Iraqi forces continue their campaign to push IS militants out of their Iraqi stronghold of Mosul.
Raqqa was named the de facto capital of the “caliphate” proclaimed by IS in 2014.
A report by IHS Conflict Monitor in October 2016 said the jihadists had lost about 16% of the territory they held at the beginning of the year, and that overall they had lost just over a quarter of the territory they controlled in January 2015.
The SDF alliance, which is dominated by the Kurdish Popular Protection Units (YPG) militia, has emerged as a key ally of the US-led coalition over the past two years, leading the fight against IS on the ground in northern Syria.
Turkey, Syria’s closest neighbour to the north, is not expected to take part. It considers the YPG a terror organisation and says it will not accept a role for the Kurds in the liberation of Raqqa.
Announcing the “Wrath of the Euphrates” operation, the SDF said they hoped Turkey would not “interfere in internal Syrian affairs”.
Of the SDF’s 30,000-strong force, up to 25,000 fighters come from the YPG, according to some estimates.
Separately on Sunday, the US army’s highest-ranking general met his Turkish counterpart in Ankara in a previously unannounced visit, the Turkish military said.
They provided no further details on the meeting between Joseph Dunford, Chairman of the US Joint Chiefs of Staff, and General Hulusi Akar.