Trump sent a letter to Iran's supreme leader Ayatollah Ali Khamenei last month calling for negotiations but warning of military action if diplomacy failed.

On Thursday, the US president said he favoured "direct talks", arguing they were "faster" and offered a better understanding than going through intermediaries.

But Iranian Foreign Minister Abbas Araghchi said direct talks made no sense with a country "that constantly threatens to resort to force in violation of the UN Charter and that expresses contradictory positions from its various officials".

"We remain committed to diplomacy and are ready to try the path of indirect negotiations," he was quoted as saying in a statement issued by his ministry.

"Iran keeps itself prepared for all possible or probable events, and just as it is serious in diplomacy and negotiations, it will also be decisive and serious in defending its national interests and sovereignty."

On Saturday, Iranian President Masoud Pezeshkian said his country was willing to engage in dialogue with the United States on an "equal footing".

He also questioned Washington's sincerity in calling for negotiations, saying "if you want negotiations, then what is the point of threatening?"

Iran and the United States have had no diplomatic relations since shortly after the 1979 Islamic Revolution with some regional countries like Oman playing a mediating role between the two sides.

- Letter diplomacy -

Trump's letter was delivered to Iran via the United Arab Emirates, and Tehran responded at the end of March via the Sultanate of Oman.

On Sunday, the chief of staff of the Iranian armed forces, General Mohammad Bagheri, said Iran's response stressed that "we seek peace in the region".

"We are not the ones who start wars, but we will respond to any threat with all our might," he said of the content of Iran's response.

Western countries, led by the United States, have for decades accused Tehran of seeking to acquire nuclear weapons.

Iran rejects the allegation and maintains that its nuclear activities exist solely for civilian purposes.

In 2015, Iran reached a landmark deal with the permanent members of the UN Security Council, namely the United States, France, China, Russia, and the United Kingdom, as well as Germany, to limit its nuclear activities.

The 2015 agreement -- known as the Joint Comprehensive Plan of Action -- gave Iran sanctions relief in exchange for curbs on its nuclear programme to guarantee that Tehran could not develop a nuclear weapon.

In 2018, during Trump's first term in office, the United States withdrew from the agreement and reinstated biting sanctions on Iran.

A year later, Iran began rolling back on its commitments under the agreement and accelerated its nuclear programme.

On Monday, Ali Larijani, a close adviser to Khamenei, warned that while Iran was not seeking nuclear weapons, it would "have no choice but to do so" in the event of an attack against it.