US President Donald Trump said he ordered the killing of Qassem Soleimani to stop a war, not start one, saying the Iranian military commander was planning imminent attacks on Americans.

"Soleimani was plotting imminent and sinister attacks on American diplomats and military personnel but we caught him in the act and terminated him," Trump told reporters at his Mar-a-Lago resort.

"We took action last night to stop a war. We did not take action to start a war," Trump said, adding that the United States does not seek regime change in Iran.