(Via CNN NEWS) Kurdish forces fired mortars and explosives at extremist militants Sunday as the battle to retake a strategic dam in northern Iraq raged on.

Fighters for the Islamic State in Iraq and Syria set up high powered explosives around buildings and detonated them, Kurdish Intelligence chief Masrour Barzani said.

Kurdish forces fought back, sparking clashes. Smoke could be seen rising in the distance.

ISIS, the extremist militant group that calls itself the Islamic State, seized the strategic Mosul Dam this month.

U.S. warplanes joined the effort to retake the dam amid growing concern it is not maintained and could rupture, a U.S. official told CNN.

Engineering studies show that a failure of the dam would be catastrophic, resulting in flooding all the way to Baghdad, the official said on condition of anonymity.

Mosul Dam is Iraq’s largest hydroelectric dam, and it sits on the Tigris Rivers about 50 kilometers (31 miles) north of the city of Mosul. ISIS fighters seized it this month following fierce fighting.

The United States estimates there may be up to 400 ISIS fighters in and around the dam complex, the official said.

The U.S. military confirmed a mix of fighter jets and drones carried out nine airstrikes near Mosul and the Kurdish regional capital of Irbil. The strikes targeted armored vehicles used by ISIS fighters, it said.

U.S. Central Command declined to provide further details, citing security of its personnel. (read more)