A Syrian rebel group which is part of a new U.S.-backed alliance that also includes the Kurdish YPG militia announced on Thursday plans for an imminent offensive against ISIS militants in Raqqa province, the extremists' stronghold in Syria.

The alliance includes the Kurdish YPG militia and a number of Syrian Arab rebel groups such as the Raqqa Revolutionaries Front, which consists mainly of Arab tribal insurgents drawn from the area.

The Arab groups in the new alliance are operating under the name "The Syrian Arab Coalition".

Carter told the Senate Armed Services Committee on Tuesday the U.S. military would intensify air strikes and might carry out more ground attacks against ISIS.

He said the United States was now supporting Syrian Arab forces already inside the country fighting IS rather than trying to train completely new moderate Syrian fighters outside the country, an effort that cost millions of dollars and produced few trained troops.

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