Cleric Ahmad Khatami told worshippers attending Eid prayers in Tehran the US President’s offer for direct talks with leaders in Iran was not acceptable.
Mr Khatami said: “The costs of a possible US war on Iran will be definitely heavy for Americans, and any aggression against Iran will inflict costs not only on America, but also on its ally, the Zionist regime.”
US ally Israel is concerned by Iran’s engagement in Syria and Mr Trump has targeted Tehran over the country’s involvement in regional affairs across Lebanon, Syria, Yemen and Iraq as well as its continued ballistic missile testing, in violation of UN resolutions.
Mr Khatami said: ”Americans say you should accept what we say in the talks. So this is not negotiation, but the dictatorship. The Islamic Republic and the Iranian nation would stand up against dictatorship.”
The US has sold hundreds of billions of dollars of weapons to Iran’s rival as they demand that Tehran cut back on its ballistic missile program.
Mr Trump’s national security adviser said on Wednesday that the US president wanted to maximise pressure on Iran.
John Bolton said: “There should not be any doubt that the United States wants this resolved peacefully, but we are fully prepared for any contingency that Iran creates”.
In May, the US announced it would abandon the 2015 Iran nuclear deal and would re-impose nuclear-related sanctions even threatening companies with penalties if they continue to operate in Iran.
The sanctions, that began in August, target US dollar financial transactions, Iran’s automotive sector and the purchase of commercial planes.
The US is trying to halt Iranian oil exports to force the country to negotiate a new nuclear agreement and to curb its influence in the Middle East.
Stronger sanctions targeting Iran’s oil sector and central bank are due to be imposed in November.
The US President has offered talks with the leaders of Iran after he imposed sanctions on Tehran after the country’s withdrawal from the 2015 Iran nuclear deal.
After Iran’s Supreme Leader Ayatollah Ali Khamenei rejected Trump’s offer of unconditional talks on a new nuclear deal, Mr Trump said: “If they want to meet, that’s fine, and if they don’t want to meet, I couldn’t care less.”