U.S.-backed forces have started a "multi-pronged" campaign to wrest control of Syria's Tabqa Dam from Daesh, the U.S.-led coalition confirmed Wednesday.

Spokesman Army Col. Joe Scrocca told reporters the Syrian Democratic Forces (SDF) offensive took Daesh by surprise when the fighters carried out a "daring air assault" behind enemy lines.

"It takes audacity and courage, and the SDF has that in spades," he said.

The coalition backed the offensive with helicopters and other aircraft to airlift the SDF, airstrikes, M777 Howitzer artillery, special operations advisors, and Apache helicopters that provided close air support.

"This is a message for ISIS: their day is done," Scrocca said, referring to the Islamic State of Iraq and Syria, another name for Daesh.

Scrocca said the airlift was the first such action taken by the coalition, and he didn't know if it would be the last.

"I’m sure ISIS will be expecting nothing less," he said.

The Tabqa Dam sits on the southeastern outlet of Lake Assad. It lies about 25 miles (40 kilometers) from the city of Raqqah, Daesh's self-declared capital that the SDF has been working to encircle.

Retaking the dam, "a key element of northern Syria’s economy", and the surrounding areas is considered a major goal in the effort to isolate Raqqah.

"The dam, city, and air fields are locations critical to ISIS’ ability to import and harbor foreign fighters, export terror, and is ISIS’ last link to their territory west of the Euphrates," said Scrocca.

He said the dam's potential destruction would create a humanitarian catastrophe, and the coalition is "taking every precaution" to make sure that does not happen.

U.S. support for the YPG-backed SDF has long vexed Turkey, who views the YPG as the Syrian offshoot of the outlawed PKK group. Turkey, the U.S. and the EU have designated the group a terrorist organization, but neither the U.S. nor EU have followed Turkey's lead on the YPG.

Asked if the SDF forces included Kurdish forces, Scrocca said they did, but he did not confirm whether the YPG had participated.

Approximately 80 percent of the SDF's forces were Arab fighters, he said.