By Rodi Said and Tom Perry
RAQQA PROVINCE, Syria/BEIRUT (Reuters) - An alliance of U.S.-backed militias started a new phase of its campaign against the Islamic State-held city of Raqqa on Saturday, aiming to complete its encirclement and sever the road to militant strongholds in Deir al-Zor province.
The Syrian Democratic Forces (SDF) said in a statement the action was being undertaken with "increasing support from the (U.S.-led) international coalition forces" through both air strikes and backing from coalition special forces on the ground.
The SDF, which includes the powerful Kurdish YPG militia, launched its multi-phased campaign in November aimed at encircling and ultimately capturing Raqqa. It is the main U.S. partner in the fight against Islamic State in Syria.
Islamic State is also being fought in a separate campaign by the Turkish army and Syrian rebel groups northeast of Aleppo, and on several fronts across Syria by the Syrian government backed by the Russian air force and Iranian-backed militia.
Jordan, part of the U.S.-led coalition, said it had conducted air strikes against Islamic State targets in southern Syria on Friday night, hitting an ammunition depot, a car bomb factory, and a barracks.
Islamic State is fighting hard to preserve its Syria foothold as it loses ground in Iraq. It still holds swathes of Syrian territory, mostly in central and eastern Syria.
A SDF commander told Reuters the forces had so far advanced a few kilometers (miles) in the latest phase, which aims to capture areas to the east of the city, including the highway linking it to Deir al-Zor province.
Deir al-Zor, which is almost entirely in Islamic State hands, stretches all the way to the Iraqi border. A Kurdish military source told Reuters on Tuesday that the goals of this phase include capturing the main highway.
Several hundred U.S. special forces soldiers have been supporting SDF operations against Islamic State in northern Syria. France said in June that its special forces were advising rebels in the same area.
Representatives of the U.S.-led coalition looked on as the statement declaring the start of the new phase was read out in a village in northern Raqqa province.
This is the third phase of the Raqqa operation. The first phase targeted areas north of Raqqa city. The second, targeting areas to the west of the city, is ongoing, with SDF forces yet to capture the Islamic State-held Euphrates dam.
Air strikes on Friday in Raqqa hit two bridges over the Euphrates river, hindering movement from the city southwards and killing six IS militants, the Syrian Observatory for Human Rights reported.
U.S. support for the SDF has been a point of tension with NATO ally Turkey, which views the YPG as an extension of the Kurdistan Workers Party (PKK), a group that has fought a three-decade insurgency in Turkey.
The United States says it is providing training and material support only to Arab elements of the SDF. It supplied them last month with armored vehicles for the first time to help in the Raqqa campaign.
U.S. President Donald Trump signed an executive order last week requesting the Pentagon, joint chiefs of staff and other agencies to submit a preliminary plan in 30 days for defeating Islamic State.
One key decision awaiting the Trump administration is whether to directly provide weapons to the YPG.
The U.S.-backed campaign against Islamic State in Syria has focused mostly on northern parts of the country. Turkey launched its own offensive against the group along the border in August, deploying its army in support of Free Syrian Army rebel groups.
The Turkish campaign, which also aims to prevent further expansion of YPG control, has been encountering fierce Islamic State resistance in al-Bab since December.
Syrian government forces have staged a rapid advance of their own towards al-Bab in the last two weeks, risking a confrontation with the Turkish army.
The Syrian Observatory for Human Rights said the Syrian army captured a town from IS to the south of the city on Saturday, while the Turkey-backed forces captured one to its east.
(Additional reporting by Suleiman al-Khalidi in Amman; Writing by Tom Perry in Beirut; Editing by Alexander Smith and Louise Heavens)