An American teenager who was forced to join Isis by her father, has spoken out about her ordeal after escaping from territory still held by the militant group.

"My father, of course, didn't tell us that we were coming to Syria," the 15-year-old told US broadcaster CBS News, on the outskirts of the Syrian city of Raqqa, which was until recently a stronghold for the militant group.

"When it was time to get out of the car, and cross the border, he was like 'you're going to Syria,' and it was a really big shock."

On their arrival, the teenager from Kansas said she was coerced into marrying another fighter.

She said her father had been killed and she is now living with several other women who have escaped Raqqa in recent weeks.

Describing conditions under the harsh rules of the so called Caliphate, she said: "We were prisoners. We were just quiet."

She added they were told: "Shut up, sit down you're in the house, be glad your head isn't chopped off."

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Raqqa is expected to fall by the end of October, according to the United Nations’ special envoy to Syria, Staffan de Mistura. It is estimated that around 4,000 civilians remain trapped in the city.

Isis once controlled vast swathes of Western Iraq and Syria, but most of this territory has been recaptured in recent months by the US-backed coalition of Kurdish fighters and Syrian Democratic Forces.

The 15 year old girl now hopes to be reunited with her mother in the US (Gabriel Chaim/ CBS News)

Another Isis stronghold, the city of Mosul in northern Iraq, was declared "liberated" on 10 July.

The girl now hopes to be reunited with her mother, who she believes is still living in the US.