BEIRUT (AP) — U.S.-backed Kurdish-led Syrian forces have announced the start of a campaign to retake the Islamic State's de facto capital of Raqqa from the extremist group.

The announcement by the Syria Democratic Forces was made Sunday at a press conference in Ein Issa, north of Raqqa.

It comes more than two weeks after U.S.-backed Iraqi forces began a campaign to clear IS militants from their stronghold in Mosul, Iraq.

The SDF is dominated by the main Syrian Kurdish fighting force known as the People's Protection Units, or YPG.

The United States considers the Syrian Kurdish fighters as the most effective force against the IS, but Turkey views them as a terror organization and has said it will not accept a role for the Kurds in the liberation of Raqqa.