A pair of arm drones heading for Erbil International Airport of Iraq was shot down, the U.S. military said on Sept. 11.

A U.S. official confirmed a counter-rocket known as the artillery and mortar system (C-RAM) struck down two explosive laden, and Iranian made, drones. One drone exploded inside the airport perimeter while another exploded outside.

No damage or casualties were reported on the 20th anniversary of 9/11. The Kurdish-held airport is in the country’s northern region with U.S. forces stationed at the facility.

“These attacks endanger the lives of civilians, and the partner forces from the Israeli Security Forces, Peshmerga, and Coalition,” Coalition spokesman Col. Wayne Marotto said on Twitter.

These kinds of attacks have been increasingly prevalent in recent months, usually targeting U.S. troops or interests across Iraq.

Similar attacks have been blamed on Iran-aligned Shi’ite Muslim militia that vowed to battle until the remaining 2,500 U.S. troops leave the country.

Erbil has already experienced four attacks this year according to RUDAW. One of the attacks occurred on July 24, with a drone targeted at a Coalition base north of the city. However, it was unsuccessful and induced no injuries or material damage, the North Press Agency reported.

The Erbil airport attack came after a two-month hiatus in drone and rocket attacks against the U.S. military in Baghdad and military bases around Iraq.

Rockets attacked Baghdad Airport on July 8. Projectiles targeted the tightly protected green zone where the U.S. Embassy is situated. It damaged property and caused no injuries.

The United States is leading an international military coalition tasked with assisting Iraqi forces in combating the Sunni terrorist Islamic State group’s remaining fractions.

America also deployed 900 troops to neighboring Middle Eastern country Syria to search for Islamic State remnants.