"This official response includes a letter in which our position regarding the current situation and Mr Trump's letter has been fully explained to the other party," Abbas Araghchi told the official IRNA news agency.

He added that the letter was delivered to Oman, which has served as an intermediary in the past in the absence of US-Iranian diplomatic relations.

He did not disclose the nature of the response nor say when it had been sent.

US Secretary of State Marco Rubio, asked by reporters on his plane over the Caribbean whether the United States has received the letter, said: "I'm not going to comment on that yet".

He said that the United States had expected there to be a response and "obviously at that point, the president will decide what steps if he any he wants to take next".

Trump, who in 2018 pulled the United States out of an agreement to relieve sanctions on Iran in return for curbs on its nuclear programme, now says he is open to diplomatic talks.

The US president revealed at the start of March that he had sent a letter to Iran's supreme leader Ayatollah Ali Khamenei.

At the same time, Trump has pushed ahead with his "maximum pressure" programme of additional sanctions on Tehran and the threat of military action if it refuses to negotiate.

"Our policy remains not to negotiate directly (with Washington) under 'maximum pressure' and the threat of military action, but indirect negotiations, such as those that took place in the past, can continue," Araghchi said.

- Intermediaries -

Iran and the United States have not had official diplomatic relations since 1980.

However, the two countries have engaged indirectly via the Swiss embassy in Tehran, which represents US interests in Iran.

Oman has also played the role of mediator in the past, as has Qatar, to a lesser extent.

Trump's letter was delivered to Iran by the United Arab Emirates.

Iran agreed to the deal curbing its nuclear programme in 2015 after negotiations with major powers including the United States and China.

Western government have for decades suspected that Tehran is pursuing a nuclear weapons capability, a charge that Iran denies, insisting that the programme is solely for civilian purposes.

Iran continued to respect the deal for a year after Trump pulled out, according to the International Atomic Energy Agency, before rolling back its commitments.

The US decision to withdraw was driven in part by Iran's ballistic missile programme, which was not covered by the deal and which Washington perceived as a threat.