U.S. forces shot down a pair of Iranian drones that attacked the Irbil airport in Kurdish-held northern Iraq late on 20th anniversary of Sept. 11.

There were no injuries or damage, according to a spokesman for the U.S.-led coalition in Iraq.

The U.S. counter-rocket, artillery and mortar system (C-RAM) engaged the two bomb-laden drones which were made in Iran, a separate U.S. official told Fox News. 

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“Each attack against the GoI, KRI and the Coalition undermines the authority of Iraqi institutions, the rule of law and Iraqi National sovereignty,” Operation Inherent Resolve spokesman Col. Wayne Marotto said in a statement posted to Twitter. “These attacks endanger the lives of civilians and the partner forces from the ISF, Peshmerga and Coalition.” 

The Erbil airport, where some U.S. troops are based, has come under a series of attacks this year by Iranian-backed militia forces.

The attack is the first following a two month lull in drone and rocket attacks to target the U.S. presence in Baghdad and military bases across Iraq. On July 8, rockets landed in and around the heavily fortified Green Zone in the Iraqi capital Baghdad, which houses the U.S. Embassy. It caused material damage but no casualties.

Until recently the attacks were a frequent occurrence. The U.S. has blamed Iran-backed militias for attacks. More recently, the attacks have become more sophisticated, with militants using drones instead of Katyusha rockets.

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Roughly 2,500 U.S. troops remain in Iraq, with an additional 900 troops in neighboring Syria to make sure the remnants of ISIS stay underground. 

The associated Press contributed to this report.