US-backed Syrian militias cut the last main road out of Islamic State-held Raqqa on Monday, severing the highway between the group's de facto capital and its stronghold of Deir al-Zor province, a Kurdish military source and a war monitor said.
It marks a major accomplishment for the US-backed Syrian Democratic Forces (SDF) in the multi-phased campaign being waged since November to encircle and ultimately capture Islamic State's base of operations in Raqqa city in northern Syria.
"Cutting the road between Raqqa and Deir al-Zor means that practically the encirclement of the Daesh (Islamic State) capital is complete by land," the Kurdish military source told Reuters, adding that the only remaining way out was south across the Euphrates River.
Air strikes by the US-led coalition have destroyed the bridges across the Euphrates to Raqqa city, the British-based monitoring group, the Syrian Observatory for Human Rights, said.
"It is a big victory but there is still a lot to accomplish," the Kurdish military source said, speaking on condition of anonymity.
The Observatory said families brought recently by Islamic State to Raqqa from areas to the west had been forced to cross the river by boat, reflecting the problem facing IS in reaching the city.
The SDF confirmed that its forces had taken the Raqqa-Deir al-Zor road.
"The road is under the control of the SDF," spokesman Talal Silo said in a voice message sent to Reuters.
There was no immediate word from the Islamic State group.
The SDF has been the main US partner in Syria, where IS still controls swathes of territory including nearly all of Deir al-Zor province which stretches to the Syrian border with Iraq.
The SDF is an alliance of militias including the Kurdish YPG and allied Arab groups. Its campaign against Islamic State in northern Syria is receiving air support from the US-led coalition and backing on the ground from coalition special forces.
The YPG's involvement in the campaign for Raqqa is a major point of contention between the US and its ally Turkey.
Turkey, with the second largest army in NATO, is adamant that Washington should switch support for the planned Raqqa offensive from the Kurdish YPG militia to Syrian rebels Turkey has trained and led against Islamic State for the past year.
(Reporting by Tom Perry; Editing by Robin Pomeroy)