US President Donald Trump said he decided to strike Iran because it was the "last and best chance" to stop the country's alleged nuclear bomb program.

"This was our last and best chance to strike, what we are doing right now, and eliminate the unbearable threats posed by this sick and sinister regime," Trump said at the White House. "The regime already had missiles capable of hitting Europe and our bases, both domestic and foreign, and would soon have had missiles capable of reaching our beautiful America," he added.

The president also said the US attack on Iran is proceeding ahead of schedule, but warned that the war could last "much longer" than his initial estimate of one month. "We are already significantly ahead of our projected timeline," Trump said at the White House, adding: "From the very beginning we planned for four to five weeks, but we have the ability to go much longer. And we will. However long it takes — that's fine. However long it takes — we are ready."

"First, we are destroying Iran's missile capabilities, and you can see this hourly, as well as their ability to produce new, fairly sophisticated missiles," Trump said. "Second, we are destroying their navy. We have already disabled 10 ships," he added. "Third, we are ensuring that the world's largest state sponsor of terrorism will never be able to obtain nuclear weapons. And finally, we are ensuring that the Iranian regime will not be able to continue arming, financing, and commanding terrorist armies beyond its borders."