Citing the escalating threat of the Islamic State, the U.S. announced it would delay delivery of eight F-16 fighter planes to Iraq, instead re-routing them to Arizona, where Iraqi pilots are participating in a U.S.-run training program, the Defense Department said.

The Iraqis purchased the F-16 fighters -- the first of its kind to be used by Iraqi Air Force pilots -- and they were headed for Balad Air Base north of Baghdad. But repeated attacks by ISIS forced the Pentagon to go with Plan B, the Defense Department said.

“The initial group of F-16s is now going to be delivered to Tucson, Arizona, where there are Iraqi pilots currently in a training pipeline,” Col. Steven Warren said during a media briefing Monday.

Warren said three of the eight planes will be delivered in December and then one per month through May for a total of eight F-16s.

“We expect the Iraqi pilots will begin flying their own aircraft for continuation training beginning in January,” he said.  “All maintenance will be provided by a contract, logistics support.”

The Iraqi pilots have been training in the United States for several months using U.S. training aircraft, Warren said.

ISIS fighters have captured large swaths of Iraq in the past few months. Though Balad Air Base is still controlled by the Iraqi government, terrorist attacks have made the base unsafe for American contractors. U.S. contractors are hired to maintain the planes on the base.

The runways at Balad are littered with debris and too damaged for regular use by fighter planes, the Wall Street Journal reported Tuesday.

Last week, President Obama authorized the deployment of up to 1,500 more American troops to bolster Iraqi forces. The plan could boost the total number of American troops in Iraq to 3,100. Currently, there are about 1,400 U.S. troops there, out of the 1,600 previously authorized.