He said the strikes were the "last, best chance" to hit Iran, an arch foe of the United States for decades.

"Our objectives are clear," he told an event at the White House after US officials gave a changing variety of reasons for the US-Israeli offensive.

"First, we're destroying Iran's missile capabilities... Second, we're annihilating their navy... Third, we're ensuring that the world's number-one sponsor of terror can never obtain a nuclear weapon.

"Finally we are ensuring the Iranian regime can't continue to arm, fund and direct terrorist armies outside of their borders," he said.

Trump has previously justified the war saying that Iran was attempting to rebuild its nuclear program and could soon have missiles able to strike the United States.

But he had not previously listed four reasons, including preventing Iran from supporting regional militant groups such Hezbollah and Hamas.

US and Israeli forces have so far struck hundreds of targets across Iran, including the Islamic republic's missiles, navy and command-and-control sites.