BEIRUT (Reuters) - U.S.-backed local forces fighting Islamic State in Syria on Friday reached one side of the Tabqa dam, one of the top prizes in their campaign to drive the jihadist group from Raqqa city, local campaign officials said.

The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias supported by a U.S.-led international coalition, is fighting Islamic State at the entrance to the dam, Jihan Sheikh Ahmed, the SDF spokeswoman for the Raqqa campaign, said.

The dam, the biggest on the Euphrates, stretches 4km across the river to the southern bank and provides one of the few land crossings remaining after the destruction of many bridges during the conflict.

Tabqa is about 40 km (25 miles) west of Raqqa, which Islamic State has used for years as one of its main bases of operations, including to plan and direct attacks overseas, and which sits along the northern bank of the Euphrates.

With the help of coalition air strikes and U.S. special forces on the ground, the SDF launched an operation late last year to isolate Raqqa and has surrounded the city in a large pocket of land backing onto the river.

Late on Tuesday, the U.S. coalition air dropped SDF forces onto the southern bank of the Euphrates west of Tabqa, part of their preparations for an assault on the dam and a nearby town and airbase of the same name.

Jean-Yves Le Drian, Defence Minister of France, which is a member of the U.S.-led coalition, said on Friday that the battle to recapture Raqqa was likely to being in the coming days.

(This version of the story corrects title of French defense minister in last paragraph)