Twenty to 25 Islamic State fighters attacked a major Iraqi base housing U.S. forces Friday, using a combination of suicide bombers and a follow-up force to attempt to penetrate the base’s perimeter, the Pentagon said.

The attack by the Islamic State of Iraq and Syria on Al Asad air base, a massive Iraqi base northwest of Baghdad where several hundred of the 2,600 U.S. troops are housed, follows continuous indirect fire attacks upon the base this week and the capture of the nearby town of al Baghdadi by ISIS earlier in the week.