U.S. Special Operations forces walk in the village of Fatisah in the northern Syrian province of Raqqa on May 25, 2016.
(Delil Souleiman/AFP via Getty Images)

Hundreds of Syrian fighters and an undisclosed number of U.S. Special Operations forces launched a large-scale heliborne assault on the Islamic State in Syria, the Pentagon said Wednesday.

The operation began Tuesday night and was focused around the area of the Tabqa Dam — a choke point on the Euphrates River that has been a key hub for the Islamic State and source of hydroelectric power for the region. The dam is about 25 miles west of the group’s de facto capital of Raqqa.

The coalition of Kurdish and Arab fighters, known as the Syrian Democratic Forces (SDF), along with their U.S. advisers, were inserted by U.S. aircraft, probably waves of V-22 Ospreys and twin-rotor CH-47 Chinooks, on the southern bank of the river behind Islamic State defenses — taking the militants by surprise.

Col. Joe Scrocca, the spokesman for the U.S.-led campaign in Iraq and Syria, told reporters that the forces did not come under fire as the troops were dropped off, but there was heavy fighting in the area on Wednesday.

The operation, according to Scrocca, could take weeks and aims to capture the dam, its surrounding town and an airfield to the south. The ground forces were backed up by helicopter gunships, Marine 155mm artillery and U.S. airstrikes, he said. While U.S. ground forces were involved, the operation doesn’t represent a significant change in their role, Scrocca said. The U.S. troops are still relegated to supporting the SDF with advice and tactical assistance. Helicopter insertions in enemy territory, known as air assaults, are inherently dangerous regardless of an “adviser” status.

The operation represents one of the most aggressive moves in the campaign to retake Raqqa from the Islamic State. Since November, SDF forces have incrementally moved toward the stronghold in an attempt to isolate it. The assault on the dam — if successful — will cut off one of the key routes into the city. Access to the Tabqa Dam will also give advancing Syrian forces unfettered access between the north and south banks of the Euphrates, presumably allowing for a dual-pronged attack into the city itself.

While the dam was still under Islamic State control Wednesday, U.S. officials were optimistic that it could be taken soon.

Leading up to the assault, the U.S.-led coalition conducted numerous airstrikes in the area. In recent days, one of the strikes appears to have hit a school that housed displaced families in the area, according to local conflict monitors. Scrocca could not immediately confirm whether U.S. aircraft had struck the building, but said the Pentagon was looking into it.

The activist group Raqqa is Being Slaughtered Silently said about 50 families had been located at the school in the village of Mansoura, while the Britain-based Syrian Observatory for Human Rights — which relies on local reports — said 33 bodies had been recovered after the bombing.

Correction: An earlier version of this story misidentified the Euphrates River as the Tigris River.

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