U.S. forces have joined an offensive behind Islamic State lines in Syria, aiming to recapture a strategically vital dam near the ISIS capital of Raqqa.

CNN describes U.S. participation in the assault as “unprecedented”:

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.

Unfortunately, ISIS is said to remain in control of the area, although Col. Scrocca said the “daring” operation caught Islamic State fighters by surprise, and they are “still reacting to this maneuver.”

The Islamic State has controlled the Tabqa dam, which provides electric power for a large region in Syria, since 2013. It is also one of the few remaining land passages across the Euphrates River as many bridges have been destroyed. Capturing it would help isolate Raqqa for a final assault, but Scrocca also voiced concerns that a “severe humanitarian crisis” would result if the dam was destroyed — something ISIS is obviously not above doing intentionally.

Reuters reported on Friday that fighters from the Syrian Democratic Forces, a Kurdish-led militia group, have reached the entrance of the dam and are fighting Islamic State militants there. Islamic State media is denying reports that the SDF is close to capturing the dam and claims ISIS fighters were able to repel the U.S.-led helicopter landing operation.