President Donald Trump on Sunday warned Iran of consequences "the likes of which few throughout history have ever suffered," if it threatens the United States.
"NEVER, EVER THREATEN THE UNITED STATES AGAIN OR YOU WILL SUFFER CONSEQUENCES THE LIKES OF WHICH FEW THROUGHOUT HISTORY HAVE EVER SUFFERED BEFORE," he said on Twitter in a direct message to Iranian President Hassan Rouhani.
"WE ARE NO LONGER A COUNTRY THAT WILL STAND FOR YOUR DEMENTED WORDS OF VIOLENCE & DEATH. BE CAUTIOUS!" Trump said, writing the entire message in capital letters.
The response came after Rouhani earlier Sunday issued his own warning to the US leader not to "play with the lion's tail," saying that conflict with Iran would be the "mother of all wars".
The high-stakes verbal sparring is reminiscent of the exchanges Trump had last year with North Korean leader Kim Jong Un, before tensions eased and the two leaders met this year in an historic summit.
Trump has made Iran a favourite target since his unexpected rapprochement with nuclear-armed North Korea.