United States president Donald Trump said Iran’s Supreme Leader Ayatollah Ali Khamenei should be very careful about what he says after Khamenei harshly criticised the United States in a Friday prayers sermon in Tehran.

“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,” Trump said in a tweet.

“Their economy is crashing, and their people are suffering. He should be very careful with his words!”

Khamenei dismissed Trump as a “clown” as tensions between the countries escalated once more.

Addressing Friday prayers in Tehran for the first time since 2012, Ayatollah Ali Khamenei accused Washington of lying when it vowed support for the Iranian people, and said the US would “stab them with their poison dagger”.

Khamenei said America had been “cowardly” when it killed Iran’s top general, Qassem Soleimani, in an airstrike earlier this month.

In response, Iran launched a barrage of ballistic missiles targeting US troops in bases in Iraq, without causing serious injuries.

With Iran’s Revolutionary Guard braced for a US counter-attack, it mistakenly shot down a Ukrainian jetliner shortly after it took off from Tehran’s airport, killing all 176 passengers on board, mostly Iranians.

Khamenei called the shooting down of the plane a “bitter accident” but insisted the Revolutionary Guard “maintained the security” of Iran.

Ayatollah Khamenei has held the top office since 1989 and has the final say on all major decisions. The 80-year-old openly wept at the funeral of General Soleimani and vowed “harsh retaliation” against the US.

Tensions between Iran and the United States have been growing since President Trump withdrew the US from Iran’s 2015 nuclear deal with world powers, which had imposed restrictions on its nuclear programme in exchange for the lifting of international sanctions.