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U.S.-allied forces in Syria launched an large-scale offensive on the ISIS stronghold of Raqqa on Tuesday, the group said.

The Syrian Democratic Forces, an umbrella group including Kurdish and Arab fighters, said on the social network that it launched a three-pronged attack on the extremists' de facto capital.

Members of what the U.S. calls the Syrian Democratic Forces gather after a training session at a firing range in northern Syria on May 21. Robert Burns / AP

SDF's commanders Rojda Felat told NBC News that 25,000 Kurdish forces were involved in the operation.

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Kurdish media organization Rudaw reported the "massive" assault also included the Kurdish People's Protection Units — known as the YPG — which is a secular Kurdish force fighting ISIS in the country.

A U.S. defense official told NBC News that Kurdish and Arab forces were still between 30 and 40 miles north of Raqqa, and just beginning to move toward the city.

The fighters will have travel through small towns and most likely encounter ISIS forces on the way, said the official who spoke on condition of anonymity.

ISIS has made Raqqa the seat of the caliphate it declared in June 2014. Situated on the Euphrates river, the northern-central city is 100 miles east of Aleppo and 130 miles west of the Iraqi border.

The offensive comes two days after an unannounced visit to Syria by Army Gen. Joseph Votel, who visited some of the 200 U.S. military advisers helping to battle ISIS, according to The Associated Press.