U.S.-backed forces now have Islamic State fighters surrounded in central Raqqa, a Syrian Kurdish commander said, but he predicted that driving the militants out could take up to four months.

"We've cleared about half of Old Raqqa ... and we're advancing on all axes," said Haval Gabar, the 25-year-old commander from the Kurdish YPG militia who is directing the assault on the Old City front in Islamic State's Syrian stronghold.

Units of the Syrian Democratic Forces, an alliance dominated by the YPG, fully linked up in Raqqa's southern districts on Tuesday, encircling the militants in the city center, which includes the Old City,