The action came as Iran has been vowing for days that it would retaliate after Trump ordered Iranian Gen. Qassem Soleimani to be killed by a drone strike last week.

The Pentagon in a statement said Iran had "launched more than a dozen ballistic missiles against U.S. military and coalition forces in Iraq," adding, "It is clear that these missiles were launched from Iran and targeted at least two Iraqi military bases hosting U.S. military and coalition personnel at Al-Assad and Irbil."

Graham said Trump should be focused on trying to "restore deterrence" and that Trump wants behavioral change from the Iranian government but not "regime change."

"This was an act of war ... by any reasonable definition," Graham, one of's top allies in the Senate, said during an interview with Fox News's. "The president has all the authority he needs under Article II to respond."

Sen.(R-S.C.) called Iran's decision to launch missiles at Iraqi bases that house U.S. military personnel "an act of war" and warned that unless Tehran backs down it will be "out of the oil business."

Graham didn't specifically outline what next steps he thinks Trump should take but said he should be focused on three areas: Iran's ballistic missile program, its support for terrorist organizations and a deal that does not allow Iran to develop a nuclear weapon.

Graham added on Tuesday that he had spoken with Trump in the wake of the Iranian strikes.

"If you're watching television in Iran, I just got off the phone with the president," he said. "Your fate is in your own hands in terms of the regime's economic viability. You continue this ... crap, you're going to wake up one day out of the oil business."

"His response tonight is to deal with those three buckets. How do we get the regime to change their behavior?" Graham said. "The price to be paid by the Iranians had to go up before they will change."Top lawmakers have been briefed by the administration on the missile launch. The House and Senate are expected to be briefed on Wednesday in back-to-back meetings that were scheduled in the wake of Soleimani's death.