Iran would have to acquire a nuclear weapon if attacked by the United States or its allies, an adviser to the country's supreme leader warned on Monday, following a threat by US President Donald Trump.

The comments came after Iran's supreme leader, Ayatollah Ali Khamenei, promised to hit back if Trump carried out a threat to bomb the Islamic republic if it did not make a deal to curb its nuclear programme.

"We are not moving towards (nuclear) weapons, but if you do something wrong in the Iranian nuclear issue, you will force Iran to move towards that because it has to defend itself," Khamenei's adviser Ali Larijani told state TV.

"Iran does not want to do this, but ... (it) will have no choice," he added.

"If at some point you (the US) move towards bombing by yourself or through Israel, you will force Iran to make a different decision."