Donald Trump has warned Iran to "never threaten the United States" during a strongly worded post on Twitter.
The US President said Iran would face its "official end" if it fights the US.
The warning comes amid heightened tensions between the US and Iran, following Mr Trump's decision a year ago to pull America out of Tehran's nuclear deal with world powers.
The US leader tweeted on Monday: "If Iran wants to fight, that will be the official end of Iran. Never threaten the United States again!"
Mr Trump's tweet came shortly after a rocket landed near the US embassy in Baghdad.
The US recently sent an aircraft carrier and bombers to the Persian Gulf, to counter what it says is a growing threat from Iran.
President Trump then addressed reports that he was ready to send 120,000 US troops to Iran in the event of an attack as "fake news", but warned he would send "a hell of a lot more troops than that" if it happened.
Last week, Britain raised the threat level for UK forces and diplomats in Iraq because of an increased risk from Iranian-backed militants.
There are around 1,400 UK military and civilian personnel supporting counter-Daesh, otherwise known as so-called Islamic State, operations in Iraq.