Since returning to the White House in January, US President Donald Trump has called for a new nuclear deal with Tehran while reinstating his "maximum pressure" policy of sanctions.

Iran has officially ruled out direct talks as long as sanctions remain, with President Masoud Pezeshkian vowing on Tuesday that his country "will not bow in humiliation to anyone."

On Friday, Trump said he had sent a letter to Iran's supreme leader Ayatollah Ali Khamenei urging negotiations and warning of possible military action if Iran refuses.

Iran's top diplomat Abbas Araghchi said on Wednesday the letter was expected to be delivered "soon" by an Arab country.

"It seems that Iran is ready for limited negotiations -- in the sense that they will not extend beyond the nuclear issue," said foreign policy analyst Rahman Ghahremanpour.

Trump appeared to be seeking a "comprehensive agreement" covering Iran's nuclear programme, missile capabilities -- long criticised by Western governments -- and its "axis of resistance", a network of militant groups opposed to Israel.

On Sunday, Iran's mission to the United Nations said Tehran might consider talks only on "the potential militarisation" of its nuclear programme.

"Should the aim be the dismantlement of Iran's peaceful nuclear program... such negotiations will never take place," it said in a statement.

- 'New expectations' -

After Trump revealed the letter, Khamenei slammed what he called "bullying" by some governments, saying negotiations with them serve only to exert dominance.

"Negotiation is a path for them, a path to set new expectations," he said in a speech on Saturday.

"It is not just the nuclear issue that they are talking about now; they are setting new expectations that Iran will definitely not meet."

Khamenei, who has final say on state matters, has warned that talks with the United States will not solve Iran's problems, citing past experience.

During his first term, which ended in 2021, Trump reimposed heavy sanctions on Iran and pulled the United States out of the 2015 nuclear deal, calling it "the worst deal ever".

Tehran began rolling back its commitments to the agreement -- formally the Joint Comprehensive Plan of Action -- a year after Washington's withdrawal. Efforts to revive the deal have since faltered.

Millions of Iranians have struggled for years under crippling sanctions, which fuelled double-digit inflation and sent the rial plunging to 930,000 against the US dollar on the black market.

Ghahremanpour said leaders in both countries are using "political rhetoric" to show "their own voter base" that they are negotiating from a position of strength.

- 'No choice' -

Ali Bigdeli, an international relations expert, said Iran appears to be "preparing, either through Russian mediation or other countries such as Saudi Arabia, to accept nuclear negotiations."

Iranian diplomats have recently held nuclear talks with Britain, France and Germany, along with separate discussions with Russia.

On Wednesday, China announced it would host three-way nuclear talks with Russia and Iran later this week.

Iran's goal, according to Ghahremanpour, was to ensure that the United States "at least refrains from imposing new sanctions to prove its goodwill".

The UN atomic agency has warned that Iran has significantly increased its stockpile of highly enriched uranium to 60 percent purity -- close to the 90 percent needed for an atomic bomb.

Iran insists its nuclear programme is purely for peaceful purposes.

Bigdeli dismissed the likelihood of military action against Iran, warning it "could set the Middle East on fire".

But he believes Iran has "no choice but to negotiate" with the United States.

"Without talks with the West and sanctions relief, we cannot overcome these economic difficulties."