US President Donald Trump said his country would hit Iran very hard in the conflict.

President Donald Trump suggested on Saturday the United States would hit Iran "very hard" and threatened to expand strikes to include new targets.

"Today Iran will be hit very hard!" Trump posted on his Truth Social media platform.

"Under serious consideration for complete destruction and certain death, because of Iran's bad behavior, are areas and groups of people that were not considered for targeting up until this moment in time."

His comments came after Iranian President Masoud Pezeshkian said on Saturday that his country would never surrender to Israel and the United States.

The United States and Israel began their campaign against Iran on 28 February, and Iran has since responded with missile and drone strikes on Israel and US interests in regional countries, mainly in the Gulf.

Pezeshkian in a speech broadcast on state TV apologised to nearby Gulf states for Iran's attacks, adding that Tehran would not target its neighbors unless attacks originated from them.

Trump in his post referenced Pezeshkian's apology, saying Iran "has apologized and surrendered to its Middle East neighbors, and promised that it will not shoot at them anymore."

"This promise was only made because of the relentless U.S. and Israeli attack," Trump wrote.

"Iran is no longer the 'Bully of the Middle East,' they are, instead, 'THE LOSER OF THE MIDDLE EAST,' and will be for many decades until they surrender or, more likely, completely collapse!"