Syria Democratic Forces enter Deir al-Zor province for first time - Kurdish military source

Syrian Democratic Forces (SDF) stand on their military vehicle in northern Deir al-Zor province ahead of an offensive against Islamic State militants, Syria February 21, 2017. REUTERS/Rodi Said

BEIRUT (Reuters) - The U.S.-backed Syria Democratic Forces (SDF) alliance has crossed into Deir al-Zor province for the first time as part of an offensive against Islamic State, a Kurdish military source said on Tuesday.

The advance into the province, most of which is under the control of the ultra-hardline group, is part of an operation to encircle and ultimately capture its de facto Syrian capital of Raqqa in the north of the country.