By Tom Perry and Humeyra Pamuk
BEIRUT/ISTANBUL (Reuters) - U.S.-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 militia spokesman said.
The development, confirmed by a British-based organisation that monitors Syria's war, marks a major blow against Islamic State, which is under intense military pressure in both Syria and Iraq.
It is losing ground to three separate campaigns in northern Syria - by the U.S.-backed Syrian Democratic Forces (SDF) militias, by the Russian-backed Syrian army, and by Turkey and allied Syrian rebels.
"Cutting the road between Raqqa and Deir al-Zor means that practically the encirclement of the Daesh (Islamic State) capital is complete by land," a Kurdish military source told Reuters, adding that the only remaining way out of the city was south across the Euphrates River.
"It is a big victory but there is still a lot to accomplish," the source said, speaking on condition of anonymity.
The SDF is an alliance of militias including the Kurdish YPG and Arab groups. It launched a campaign in November to encircle and ultimately capture Islamic State's base of operations in Raqqa city, with air strikes and special forces support from a U.S.-led coalition.
Further west, the Syrian army has made its own, rapid progress against Islamic State in the past few weeks, advancing east from Aleppo city towards the Euphrates. The Syrian army last week captured the ancient city of Palmyra from Islamic State - an operation Russia said it had planned and overseen.
A Syrian military source told Reuters the army would press on to reach the jihadists' main strongholds in Raqqa and Deir al-Zor.
Syrian government forces, meanwhile, have taken over positions from a U.S.-backed militia in the northern city of Manbij on a frontline with Turkish-backed rebel forces, in line with a deal brokered by Russia, the militia's spokesman said on Monday.
Last week, Turkish President Tayyip Erdogan said Manbij would be the next target in the campaign Turkey is waging alongside Syrian rebels in northern Syria against both Islamic State and the Kurdish YPG militia.
Turkish Prime Minister Binali Yildirim said on Monday Ankara was not planning a military campaign without coordinating with the United States and Russia.
The U.S. military has also deployed a small number of forces in and around Manbij to ensure that the different parties in the area do not attack each other, a Pentagon spokesman said.
Islamic State still controls swathes of Syria, including much of the centre and nearly all the eastern province of Deir al-Zor stretching all the way to the Iraqi part of its self-declared caliphate.
The SDF has been the main U.S. partner against Islamic State in Syria. "The road is under the control of the SDF," spokesman Talal Silo said in a voice message sent to Reuters. "The road between Raqqa and Deir al-Zor."
There was no immediate word from Islamic State on the social media channels it uses to communicate news.
Air strikes by the U.S.-led coalition have destroyed the bridges across the Euphrates to Raqqa city, the British-based monitoring group, the Syrian Observatory for Human Rights, said.
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 Islamic State in reaching the city.
In Iraq, U.S.-backed Iraqi forces captured the second of Mosul's five bridges on Monday, part of a major onslaught that began in October to take back the city lost to Islamic State in 2014.
EYES ON RAQQA
The Syrian army's advance towards the Euphrates River from Aleppo has added to the pressure on Islamic State.
One of the targets of the army's advance appears to be to secure the water supply to Aleppo, which is pumped from the village of al-Khafsa on the western bank of the Euphrates.
The Observatory said on Monday the army had advanced to within 8 km (5 miles) of al-Khafsa.
The Syrian army push has also had the effect of deterring further advances south by Turkish forces and Free Syrian Army (FSA) rebels as they carve out an effective buffer zone near the border in areas seized from Islamic State.
"The army will not stop in its military operations against Daesh, and will certainly reach its most important strongholds in Raqqa and Deir al-Zor. This is a national Syrian decision," the Syrian military source told Reuters.
The source said the army was advancing at a rapid pace and there was "great dysfunction" in Islamic State's leadership.
Turkey, a NATO member, wants its Syrian rebel allies to lead the Raqqa offensive, and U.S. support for the YPG is a major point of contention between the two states.
(Reporting by Tom Perry; Editing by Mark Trevelyan and Alison Williams)