BEIRUT (Reuters) - U.S.-backed Syrian militias are expected to attack soon an Islamic State-held dam and air base west of the Syrian city of Raqqa, their spokesman said on Thursday, part of an accelerating campaign to capture the hardline group's stronghold.

The U.S.-led coalition against Islamic State airdropped Syrian Democratic Forces (SDF) fighters some 15 km (10 miles) from the dam at the town of Tabqa on Tuesday night. The SDF established a base from which to advance on the Tabqa area.

Tabqa is around 40 km (25 miles) west of Raqqa. Islamic State captured the Tabqa Dam, also known as the Euphrates Dam, and the nearby air base at the height of its expansion in Syria and Iraq in 2014.

SDF spokesman Talal Silo said the SDF forces had yet to reach Tabqa town or the nearby airport and dam. He described these as three strategically important targets and said the SDF planned to repair and use the airport once it was captured.

"Today or at most tomorrow there will be an attack operation on these three areas," he said in a phone interview.

He said that once captured, the air base runway would be repaired and "in the near future our forces will use this airport."

"In all the areas under our control, we do not have an airport that can receive planes," he said. "The coalition planes will benefit from the airport."

The SDF has been working since November to encircle Raqqa city with support from U.S. special forces and air strikes by the U.S.-led coalition against Islamic State. The SDF comprises the powerful Kurdish YPG militia and allied Arab fighters.

The campaign appears to be gathering pace as an overlapping, U.S.-backed effort in Iraq is drawing closer to driving Islamic State from Mosul.

The YPG controls swathes of northern Syria, including nearly all of the cities of Hasaka and Qamishli, home to an airport that remains under Syrian government control.