BEIRUT (Reuters) - A U.S.-backed alliance of Syrian Kurds and Arab rebel groups, supported by U.S. coalition planes, captured a dam on Saturday from Islamic State, cutting one of its main supply routes across the Euphrates, an alliance spokesman said.

Colonel Talal Selo said the seizure of the Tishrin dam helps isolate the militants’ strongholds in northern Aleppo from their territories east of the Euphrates river, where Raqqa city, their de facto capital, is located.

Selo said the rapid advance overnight by thousands of troops from the Democratic Forces of Syria had brought the dam upstream from Raqqa under their control on Saturday afternoon.

Since the U.S.-backed alliance was formed last October, its fighters have opened several major offensives against Islamic State with the ultimate goal of capturing Raqqa.

Selo said the alliance troops had cut one main IS supply line from the towns of Bab and Minbij in the northern Aleppo countryside held by the militants on the road to Raqqa city, he added.

The intensive aerial bombing by a U.S.-led coalition was instrumental in the rapid advances since the campaign started four days ago, he said.

“The coalition jets participated in the previous campaigns and this one and future ones, too. We have a strategic partnership in which they provide the air cover and we provide the troops,” he told Reuters.

Selo said the militants suffered heavy casualties in the latest round of fighting which involved several thousand fighters from the Democratic Forces of Syria with heavy armor.

The alliance troops includes the Kurdish YPG militia, various Arab groups including Jaysh al-Thuwwar (Army of Rebels) and the Arab tribal Jaysh al-Sanadeed, and an Assyrian Christian group.

They secured a stretch of territory that extended at least 20kms with scores of villages seized, he added.

“We no longer think as we did before that Daesh cannot be defeated,” the spokesman said, using a derogatory Arab name used for Islamic State.

He did not say when a major assault on Raqqa would begin.

Washington’s strategy in Syria has shifted this year from trying to train thousands of fighters outside the country to supplying groups headed by U.S.-vetted commanders.

U.S. officials said delivery of weapons since the alliance was set up would help the fighters push further south into Islamic State-held territory.

The U.S. military estimates the Democratic Forces of Syria has captured around 1,000 square kilometers of terrain in the past six weeks or so, bolstered by coalition air strikes.