Iran's Supreme Leader Ali Khamenei vowed a "firm response" on Monday if his country is attacked, after US President Donald Trump threatened to "strike Iran" if no agreement is reached with it on its nuclear program.

"They are threatening to inflict harm... but if that happens, they will certainly receive a firm response" from Iran, Khamenei said in his Eid al-Fitr sermon, referring to Trump.

While Khamenei did not explicitly mention Trump, his speech appeared to be a response to the threats the US president has issued in recent days.

In an interview with an NBC reporter, Trump said, "If they (the Iranians) don't sign a deal, there will be bombing," also threatening to punish Iran with what he called "secondary tariffs."

Trump's statements reflect an escalation after he threatened days earlier that "very bad things will happen to Iran" if it refused to negotiate a new nuclear agreement.

It is unclear whether Trump is threatening to strike Iran with US aircraft or launch an operation coordinated with Israel.

During Trump's first term, the United States withdrew in 2018 from the 2015 international nuclear agreement with Iran.

During his second term, Trump announced that he was open to talks on a new agreement with Iran, which could reduce the risk of military escalation.