The Iraqi military has used the F-16 fighter jet in combat operations for the first time, more than a year after Iraqi officials began pressing Washington to deliver them to assist in the fight against Islamic State militants.

Defense officials in Iraq and Washington on Sunday confirmed the operations, which should significantly upgrade the Iraqi military’s ability to strike Islamic State militants in coming months. Iraqi Lt. Gen. Anwar Hama Amin told media outlets in Baghdad that the Iraqi military had carried out 15 airstrikes using the fighters in the past four days, striking north of the capital city in Salaheddin and Kirkuk provinces.

Iraq had reached a multibillion-dollar agreement to buy 36 of the supersonic aircraft from the United States. The first four fighters, made by Lockheed Martin of Forth Worth, arrived at Balad Air Base in July.

“The United States is committed to building a strategic partnership with Iraq and the Iraqi people and we will continue to work with the Government of Iraq on the delivery of the remaining aircraft as they become available within the framework of the production schedule,” Pentagon spokesman Peter Cook said in a statement Sunday.

Iraqi officials began pressing for the fighters shortly after the Islamic State swept across the northern section of the country in summer 2014. But a variety of problems complicated Washington delivering them to Baghdad.

Among them, no Iraqi pilots were trained to fly the advanced aircraft and Balad Air Base was not considered secure at the time. The full range of missiles and bombs that Baghdad planned to use on the aircraft also could not be installed until they arrived in Iraq, U.S. defense officials said at the time.

The U.S. military trained Iraqi F-16 pilots in Arizona. Iraqi Brig. Gen. Rafid Mohammed Hasan died in June when his fighter went down in training near Douglas, Arizona.

Lukman Faily, the Iraqi ambassador to the United States, said in July that the arrival of the first Iraqi F-16s was a critical moment in the country’s ability to take on the militants.

“As Iraqi security forces, popular volunteers and local tribal fighters begin a major offensive to liberate key towns in Anbar, the arrival of the F-16 jets provides a much-needed boost to our air power capabilities that will allow us to target Daesh bomb-making factories and terrorist training camps,” he said using the local name for the militants.