Ebrahim Raisi speaks after casting his ballot at a polling station in Tehran, Iran, June 18, 2021. Photo: Xinhua

Iran's supreme leader on Wednesday declared Tehran would not accept Washington's "stubborn" demands in talks to revive a 2015 nuclear deal and said the United States had failed to guarantee that it would never abandon the pact again.

"The Americans acted completely cowardly and maliciously," state TV quoted Ayatollah Ali Khamenei as saying. "They once violated the nuclear deal at no cost by exiting it. Now they explicitly say that they cannot give guarantees that it would not happen again."

A US State Department spokesperson said in a statement that President Joe Biden's administration "has been sincere and steadfast in pursuing a path of meaningful diplomacy to achieve a mutual return to compliance" with the agreement. Since April 9, Tehran and six world powers have been in talks to revive the nuclear pact ditched three years ago by then US President Donald Trump, who argued it favored Iran.

The sixth round of indirect talks between Tehran and Washington adjourned on June 20, two days after hardline cleric Ebrahim Raisi was elected president of the Islamic Republic. Parties involved in the negotiations have yet to announce when the next round of negotiations will resume.

The State Department spokesperson said the US has made clear it is prepared to return to Vienna to resume negotiations. "We urge Iran to return to the negotiations soon so that we can seek to conclude this deal," the spokesperson said.

Like Khamenei, Raisi has backed the revival of the nuclear pact but officials have said that his government might adopt "a hardline" approach. Khamenei, not the president, has the last say on Iran's state matters, including the nuclear policy.

Iranian and Western officials have said significant gaps remained to reinstate the deal, under which Iran agreed to curb its nuclear program to make it harder to obtain fissile material for a weapon in return for relief from tough sanctions. Iran says it has never sought nuclear weapons and never would.

Harsh sanctions reimposed by Trump since 2018 have prompted Tehran to violate the deal's limits.