By Ellen Francis

BEIRUT (Reuters) – U.S.-backed Syrian militias said they fully seized the town of Tabqa and Syria’s largest dam from Islamic State on Wednesday, a major objective as they prepare to launch an assault on Raqqa, the jihadists’ biggest urban stronghold.

The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab fighters, have been battling the militant group for weeks in Tabqa, some 40 km (25 miles) west of Raqqa, along the Euphrates River.

With air strikes and special forces from the U.S.-led coalition, the SDF are advancing on Raqqa to ultimately take the city, which is also the Islamic State’s base of operations in Syria.

They captured Tabqa “thanks to the sacrifices of the SDF’s heroes and with the full, unlimited support of the U.S.-led international coalition”, said SDF spokesman Talal Silo.

Nasser Haj Mansour, an adviser to the SDF, said the town and the adjacent Tabqa dam were now “completely liberated” after the SDF drove all Islamic State militants out.

Brett McGurk, the U.S.’ special presidential envoy for the global coalition to counter Islamic State, confirmed on Twitter that Tabqa had been retaken.

“Confirmed: #ISIS defeated in #Tabqa Dam and Tabqa City, now in hands of Syrian Democratic Forces, led by its Syrian Arab Coalition. #SDF”, McGurk tweeted.

The Raqqa campaign appeared to have stalled around Tabqa, where the SDF made only slow progress after besieging the city. They pushed into Tabqa nearly two weeks ago, capturing most of its districts and encircling Islamic State at the dam.

The battle for Tabqa began after U.S. forces helped SDF fighters conduct an airborne landing on the southern bank of the Euphrates in late March, allowing them to gain control of an important nearby airbase.

Despite fierce objections from NATO ally Turkey, the United States this week approved supplying arms to the powerful Kurdish YPG militia, a key component of the SDF and their campaign.

Ankara strongly opposes U.S. support of the YPG, viewing it as the Syrian extension of the outlawed Kurdistan Workers Party (PKK), which has waged a three-decade insurgency within Turkey.

But the YPG has emerged as a valuable partner for the United States in the fight against Islamic State in northern Syria. Washington maintains arming the Syrian Kurdish forces is necessary to capture Raqqa, which Islamic State has also used a hub for planning attacks abroad.

Raqqa now lies in an Islamic State enclave on the northern bank of the Euphrates, after the SDF has closed in on the city from the north, east and west in recent months.

Islamic State’s only means of crossing to its main territory south of the river is by boat after air strikes knocked the area’s bridges out of service.

The jihadist group still controls swathes of Syria’s vast eastern deserts and most of Deir al-Zor province near the border with Iraq, but it has lost tracts of its territory over the past year.

(Reporting by Ellen Francis, additional reporting by Idrees Ali; editing by Robin Pomeroy, Alison Williams and G Crosse)