The US-backed Syria Democratic Forces (SDF) alliance has crossed into Deir Az Zor province for the first time as part of an offensive against ISIL, a Kurdish military source said on Tuesday.
The advance into the province, most of which is under the control of the Islamic State of Iraq and the Levant (ISIL, also known as ISIS), 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 ISIL's supply lines from Raqqa to Deir Az Zor province.
The move also expands the SDF's area of operations against ISIL, which is being fought by all sides in the complex Syria conflict.
Later on Tuesday, the Syrian Observatory for Human Rights (SOHR), a UK-based monitoring group, reported that at least 11 people were killed and more than 35 wounded in air strikes in a town in northern Deir Az Zor province, in an area where US-led coalition warplanes have operated.
It was not clear whose air force had carried out the strikes, 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 Az Zor], from the north - so, via southern Hasaka (province)," the Kurdish military source told Reuters news agency.
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The SDF, which includes the Kurdish YPG militia and other Arab fighting groups, captured some 15 villages from ISIL fighters in their incursion into the province, the source added, without specifying when this had taken place.
ISIL controls most of Deir Az Zor province apart from a Syrian government-held enclave in Deir Az Zor city and a nearby military airbase.
Different groups in Syria's multi-sided conflict are fighting a number of separate battles against ISIL.
Syria's army and its allies, backed by Russia, have been fighting back against ISIL assaults in Deir Az Zor city and have engaged in clashes with the group further west.
Turkish-backed Syrian rebels, meanwhile, are fighting for control of the northern city of Al Bab, which monitors say is still mostly in ISIL 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 that 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 UN's humanitarian coordination body said in a statement.
The air strikes in the town of al-Sur in Deir Az Zor on Tuesday hit a garage, petrol station and commercial area, the Observatory reported.
The UN-led talks on Syria's future will resume in Geneva on Thursday after UN envoy Staffan de Mistura broke them off almost nine months ago following several rounds that led ultimately to an escalation of violence.
De Mistura's office said that the focus of the meeting will be on governance, a new constitution and elections.
It stressed that "the entire agenda for the Geneva negotiations will be framed by the broader corpus of Security Council resolutions".
A spokesperson for the UN said on Friday that de Mistura was still finalising who would come to the meeting, but there were already positive responses to invitations that had gone out.
Thursday's talks in Geneva will follow last month's Russian-sponsored parallel peace talks on Syria in the capital of Kazakhstan, Astana.
Those talks ended with no joint communique, usually the minimum outcome of any diplomatic negotiation, and saw opposing Syrian groups trading angry accusations.
Russia proposed the intra-Syrian negotiations in coordination with Turkey and Iran to reinforce a shaky ceasefire.
Five years since the civil war began, more than 450,000 Syrians have been killed in the fighting, more than a million injured and over 12 million Syrians - half the country's prewar population - have been displaced from their homes.
Source: News agencies