The U.S. military and allied forces started striking ISIS targets inside Syria late Monday night.
About 20 targets have been hit, USA Today reports, including several buildings that serve as headquarters for militants.
Pentagon Press Secretary Rear Admiral John Kirby confirmed that U.S. military and partner nation forces are "undertaking military action" against ISIS militants in Syria using fighter jets, unmanned aerial drones, and ship-based Tomahawk Land Attack Missiles.
Kirby said that the decision to conduct the strikes was made by U.S. Central Command earlier Monday.
UPDATE: A diplomatic source told ABC News that Jordan, Saudi Arabia, Bahrain, Qatar, and the United Arab Emirates are involved in the operation. Many of the sites that have been targeted so far are in the ISIS stronghold of Raqqa.