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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

(reuters_tickers)

By Tom Perry and John Davison

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.

One aim of the campaign is to cut IS's supply lines from Raqqa to Deir al-Zor province.

The move also expands the SDF's area of operations against Islamic State, which is being fought by all sides in the complex Syria conflict.

Later on Tuesday, the Syrian Observatory for Human Rights monitoring group reported that at least 11 people were killed and more than 35 wounded in air strikes in a town in northern Deir al-Zor province, in an area where U.S.-led coalition warplanes have operated.

It was not clear whose air force had carried them out, or if the incident was linked to the SDF advance, the Observatory said.

"Military operations of the SDF are now taking place within the provincial boundaries of Deir al-Zor, from the north - so, via southern Hasaka (province)," the Kurdish military source told Reuters.

The SDF, which includes the Kurdish YPG militia and Arab fighting groups, captured some 15 villages from Islamic State militants in their incursion into the province, the source added, without specifying when this had taken place.

Islamic State controls most of Deir al-Zor province apart from a Syrian government-held enclave in Deir al-Zor city and a nearby military air base.

Different groups in Syria's multi-sided conflict are fighting a number of separate battles against Islamic State.

Syria's army and its allies, backed by Russia, have been fighting back against Islamic State assaults in Deir al-Zor city and have engaged in clashes with the group further west.

HUMANITARIAN CONCERNS

Turkish-backed Syrian rebels, meanwhile, are fighting for control of the northern city of al-Bab, which monitors say is still mostly in Islamic State hands, but which the rebels have pushed into.

That battle has brought the Turkish-backed rebels into close proximity with Syrian government forces, which had advanced towards the city from another direction before the rebels entered it.

The Syrian army's advance towards al-Bab has raised fears of sparking a confrontation with Turkey.

The United Nations said on Tuesday an estimated 5,000 civilians were trapped by fighting in and around al-Bab and that 300 non-combatants had been killed since December, many of them by air strikes.

Turkey and Russia have both been carrying out air strikes around the city.

"As the operation advances, parties to the conflict may be preparing for urban battles ... which could place civilians in the area at heightened risk of death and injury, as well as (making them) vulnerable to being used as human shields," the U.N.'s humanitarian coordination body said in a statement.

The air strikes in the town of al-Sur in Deir al-Zor on Tuesday hit a garage, petrol station and commercial area, the Observatory reported.

(Editing by Ralph Boulton and Robin Pomeroy)

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