US President Donald Trump has threatened that Iran will be bombed if it persists in developing nuclear weapons.

"If they don’t make a deal, there will be bombing," NBC News said the President told one of its correspondents in an interview late Saturday. It said he also threatened to punish Iran with what he called "secondary tariffs."

Trump's language represented a sharpening of his comment a few days earlier that if Tehran refused to negotiate a new nuclear agreement, "bad, bad things are going to happen to Iran."

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

Israeli Foreign Minister Gideon Saar told Politico late last month that "in order to stop a nuclear Iranian program before it will be weaponized, a reliable military option should be on the table." 

Analysts have said Iran may be just weeks away from producing a deliverable nuclear weapon — though Tehran denies it is building such arms. Either way, such an attack carries a risk of spreading to a wider conflict.

Trump in 2018 pulled the United States out of an agreement to relieve sanctions on Iran in return for curbs on its nuclear program. 

Now, in his second term, he has said he is open to talks on a new deal that could reduce the risk of military escalation.

Trump revealed in early March that he had sent a letter proposing such talks to Iran's supreme leader, Ayatollah Ali Khamenei.

Meantime, he has pushed ahead with his "maximum pressure" program of additional sanctions and the threat of military action. 

Tehran, deeply suspicious of the US administration after Trump's withdrawal from the original nuclear deal, has refused to negotiate directly with Washington.

According to NBC, Trump also said US and Iranian officials were "talking," but he did not give details.

In a video published early Sunday by Iranian state media, President Masoud Pezeshkian said that Tehran's response to Trump, delivered to intermediaries in Oman by Foreign Minister Abbas Araghchi, said that "direct negotiations were rejected."

But, he added, "The supreme leader has also emphasized that indirect negotiations can continue."

"We do not avoid negotiations," said Pezeshkian, a reformist who has called for reviving nuclear talks with the US.

"Rather, it is their unfaithfulness that has caused problems for us so far. They must prove that they can establish trust regarding decisions, and I hope this will materialize."

It was unclear whether Trump had spoken to NBC before or after Pezeshkian made his comments.

Oman has in the past served as an intermediary between the US and Iran.