U.S.-backed forces fighting ISIS enter Raqqa

The Kurdish and Arab forces backed by the United States have entered Islamic State's de facto capital of Raqqa, in an apparent bid to reclaim the militants' largest stronghold in Syria.

The Syrian Democratic Forces (SDF), a coalition of Kurdish and Arab fighters, breached the outer city limits of Raqqa, seizing a 1,000-year-old fortress in the west and a neighbourhood on the eastern side of the city, The Guardian reports.

"Raqqa to them is the capital of the caliphate, and they [Isis] have fortified it to a great extent," Nouri Mahmoud, a spokesman for the People's Protection Units (YPG), the Kurdish militia, told the Guardian.

If successful, the operation would deprive the ISIS of its second largest city, with a simultaneous campaign to take back Mosul in Iraq ongoing.

It would be a major blow to the militants' efforts to take advantage of chaos in Iraq and Syria, three years after it declared a caliphate across the borders of both nations.