"They threaten to do mischief," Khamenei said of Trump's latest threat, during a speech on Eid al-Fitr, the holiday marking the end of the Muslim fasting month of Ramadan.

"If it is carried out, they will definitely receive a strong counterattack."

In an interview on Saturday, Trump said Iran will be bombed if it does not reach a deal on its nuclear programme.

"If they don't make a deal, there will be bombing," he said, according to NBC News, which said he also threatened to punish Iran with what he called "secondary tariffs".

It was not clear whether Trump was threatening bombing by US planes alone or perhaps in an operation coordinated with Israel.

Since taking office in January, Trump has reinstated his "maximum pressure" campaign on Iran.

Under that policy in his first term as president, Trump withdrew the United States from a landmark agreement on Iran's nuclear programme in 2018 and reimposed biting sanctions on Tehran.

Western countries including the United States have long accused Iran of pursuing a nuclear weapon, which Tehran has denied, insisting its enrichment activities were solely for peaceful purposes.

- Open to 'indirect' talks -

The deal, sealed in 2015 between Tehran and world powers, required Iran to limit its nuclear ambitions in exchange for sanctions relief.

On March 7, Trump said he had written to Khamenei to call for nuclear negotiations and warn of possible military action if Tehran refused.

The letter was delivered to Tehran on March 12 by UAE presidential adviser Anwar Gargash.

On Thursday, Iranian Foreign Minister Abbas Araghchi said the country has delivered to Oman the response to Trump's letter, without delineating its content.

Araghchi maintained Iran's position of not seeking to directly engage in negotiations with the United States "under maximum pressure and the threat of military action", but left open the door for "indirect negotiations".

Oman has served as an intermediary in the past, in the absence of US-Iranian diplomatic relations severed after the 1979 Islamic revolution.

The West also accuses Iran of using proxy forces widely proscribed as terrorist organisations to expand its influence in the region, a charge Iran denies.

Iran leads the so-called "axis of resistance" against Israel, which includes Hamas in the Palestinian territories, Hezbollah in Lebanon, the Huthi rebels in Yemen and armed groups in Iraq.

"There is only one proxy force in this region, and that is the corrupt usurper Zionist regime," Khamenei said, calling for Israel to be "eradicated".

Iran does not recognise Israel, its arch enemy and the United States' main ally in the region, and frequently calls for attacks against it.