Story highlights The US is backing Syrian Democratic Forces and Syrian Arab Forces

Retaking the dam is considered a vital step

Washington (CNN) Fighters in Syria have begun a major ground offensive, backed up by US forces, to retake a vital dam near Raqqa, Syria, from ISIS, a US official told CNN Wednesday.

The attack also included an unprecedented air assault involving US helicopters landing behind enemy lines -- flying about 500 local US allies and coalition military advisers across the Euphrates River and Lake Assad so they could attack the ISIS-controlled dam and neighboring town and airfield from the south.

"This is the first time we have conducted an air movement, air assault, with the Syrian Arab Coalition and Syrian Democratic Forces," Col. Joseph Scrocca, a coalition public affairs officer, said. On a call with reporters from Baghdad Wednesday, Scrocca called the air operation a "daring assault behind enemy lines."

Scrocca would not say how many troops were involved, but a military source said about 500 troops were airlifted behind enemy lines. The attack was backed by US Marines firing M777 howitzers and close air support, including airstrikes carried out by Apache helicopters.

But ISIS remains in control of the area after the attack, and weeks of tough fighting could remain, Scrocca said.

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