The Pentagon confirmed US air assets have been used to airlift Syrian militia fighters in a bid to take the strategic Tabqa Dam, west of Raqqa, the Islamic State “capital” that is finding itself increasingly under siege.

US air assets had been used to airlift members of the Syrian Arab Coalition, part of the Syrian Democratic Forces group, in a bid to retake the Tabqa Dam, Major Adrian Rankine-Galloway, a Pentagon spokesman, told Reuters.

US forces also provided fire support, Rankine-Galloway said.

Military officials confirmed Wednesday the coalition airlift included apache helicopters, marine artillery and special operation troops.

The Tabqa Dam has strategic significance because it provides electricity to Raqqa as well as the nearby military airfield.

DEVELOPING: US Marines provide airlift & fire support to Syrian militia forces poised to take Taqba dam near Raqqa https://t.co/F1UnkCwC4epic.twitter.com/XpxflKtiNy — Andrew deGrandpre (@adegrandpre) March 22, 2017

Speaking to reporters on Tuesday, Pentagon spokesman Eric Pohan said US military advisers were helping to coordinate the operation.

He said at the time no US troops were doing any front-line fighting and declined to say how many US troops are involved and what kind of aircraft were used to ferry the Syrian Arab fighters to the area.

Tabqa Dam is “significant as a strategic target,” Pohan said, and that if successful, the operation would “basically cut ISIS off” from western approaches to Raqqa, the group’s self-styled capital. The US-backed SDF fighters have already cut off eastern approaches to Raqqa north of the Euphrates.

SDF liberate Abu Hoira village from IS near Tabqa pic.twitter.com/bojxiHTEay — لوميس روبيرتو (@loomisroberto) March 22, 2017

Meanwhile, a London-based group that monitors the Syrian conflict and a shadowy social media group that claims to speak for residents of Raqqa both said that coalition activity has caused civilian casualties.

The Raqqa is Being Slaughtered Silently group said 500 US and Syrian Kurdish forces were deployed in the Tabqa operation as of Wednesday morning. The group said dozens of civilians were dead and still missing after an airstrike on a shelter for the displaced in an Islamic State-held village in northern Syria.

Pentagon will aid in investigation of reports of coalition airstrike that killed 30+ civilians in #Syriahttps://t.co/FJ9wFRTln8 — RT (@RT_com) March 22, 2017

The UK-based Syrian Observatory for Human Rights said that at least 33 people were killed in an air strike that hit a school sheltering displaced people near Raqqa.

The Pentagon said there were no indications that US-led airstrikes hit civilians.