US President Donald Trump said Iran's supreme leader Ayatollah Ali Khamenei should be careful about what he says after he harshly criticised the US in a Friday prayers sermon in Tehran.
Mr Trump took to Twitter to rebuke Mr Khamenei for the "nasty things" he said about western powers during the sermon.
"The so-called 'Supreme Leader' of Iran, who has not been so Supreme lately, had some nasty things to say about the United States and Europe," Mr Trump said.
"Their economy is crashing, and their people are suffering. He should be very careful with his words!"
Mr Khamenei described Mr Trump as a "clown" who pretends to support the Iranian people but will "push a poisonous dagger" in their back.
In his first Friday sermon in Tehran for eight years, which aimed to address domestic anger over the accidental shooting down of a passenger plane, Mr Khamenei said Iran "slapped the US in the face" with a series of missile strikes on Iraqi bases housing US soldiers.
"The fact that Iran has the power to give such a slap to a world power shows the hand of God," Mr Khamenei, 80, said.
His comments were met by chants of "Death to America".
Brian Hook, the US Special representative for Iran, said threats made Mr Khamenei towards the US would only serve to further isolate Tehran.
"As long as the regime threatens the world it will become more isolated. Until Iran behaves like a normal nation its isolation will only deepen," Mr Hook said.
In his sermon, Mr Khamenei addressed the "tragic, very sad incident" when the Iranian military shot down Ukraine International Airlines Flight 752, killing 176 people.
He said Iran's enemies were using the incident to weaken the grip of the Islamic Revolutionary Guards Corps and to overshadow the US air strike that killed top Iranian military commander Qassem Suleimani in Baghdad.
"The plane crash was a bitter accident. It burned through our heart," Mr Khamenei said.
"But some tried to ... portray it in a way to forget the great martyrdom and sacrifice" of Suleimani, the head of the Quds Force, the foreign operations arm of the IRGC.
Mr Khamenei said Iranians and thousands of Iraqis mourned Suliemani and described Quds Force as "fighters without borders" and a "humanitarian organisation with human values".
Mr Khamenei said the IRGC "maintains Iranian security".
Suleimani's killing on January 3 triggered an outpouring of grief from Iranians. Mr Khamenei openly wept at the funeral and vowed “harsh retaliation” against the US.
But public anger against the Iranian regime on January 11 when it admitted shooting down the passenger plane, sparking three days of protests that were suppressed with the use of tear gas and live ammunition.
In his sermon, Mr Khamenei called for unity and a high turnout in a February election.
Abroad, the Iranian government and military have been accused of attempting to cover up the incident after waiting three days to admit it shot down the aircraft minutes after it took off from Imam Khomeini International Airport. At least 57 Canadians, 11 Ukrainians and 10 Swedes were among those killed, as well as four Afghans and four British citizens.
Foreign ministers of those five countries met in London on Thursday and called for full and continuing co-operation from Iran in an investigation into the incident.
The ministers also called for Iran to take the first step in offering compensation.
Iran also faces the prospect of more international sanctions as its economy reels from increasingly stringent restrictions imposed by Mr Trump since he pulled the US out of a nuclear deal between Iran and world powers.
After Suleimani was killed, Iran said it would no longer be bound by the limitations in the 2015 agreement, which imposed restrictions on its nuclear programme in exchange for the lifting of international sanctions.
European countries have been trying to salvage the deal and this week by invoked a dispute mechanism aimed at bringing Iran back into compliance. The mechanism could result in a re-imposition of sanctions.
Mr Khamenei said France, Germany and Britain were serving US interests and could be trusted.
"The iron hands of European negotiators are hidden under velvet gloves," he said.
Mr Khamenei, who has held the country's top office since 1989 and has the final say on all major decisions, was always sceptical of the nuclear agreement, arguing the US could not be trusted.
But he allowed Iranian President Hassan Rouhani, a relative moderate, to conclude the agreement with Barack Obama, the US president at the time. After Mr Trump pulled the US out of the pact, Mr Khamenei said there could be no negotiations with Washington.
Mr Khamenei last delivered a Friday sermon in February 2012, when he called Israel a “cancerous tumor” and pledged to support anyone who confronted it. He also warned against any US strikes on Iran over its nuclear programme, saying the US would be damaged “10 times over”.