Blinken says Iran nuclear talks 'cannot go on indefinitely'

US Secretary of State Antony Blinken has said that nuclear talks with Iran "cannot go on indefinitely" but that Washington is "fully prepared" to continue negotiations.

"We're committed to diplomacy, but this process cannot go on indefinitely... we look to see what Iran is ready to do or not ready to do and remain fully prepared to return to Vienna to continue negotiations," Blinken said during a visit to Kuwait on Thursday.

"The ball remains in Iran's court."

The US is indirectly involved in Iran's talks with world powers to revive a nuclear deal that gave Iran some relief from international sanctions in exchange for limits on its nuclear programme.

The deal was torpedoed in 2018 by then US president Donald Trump, who unilaterally withdrew from the agreement and imposed punishing sanctions.

Khamenei blames 'cowardly' US for pause in talks

On Wednesday, Iran's supreme leader 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."

The sixth round of indirect talks between Tehran and Washington adjourned on June 20, two days after 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.

Iranian President Hassan Rouhani's government has been holding talks with major powers in Vienna since April on bringing Washington back into the agreement.

But a deal now seems unlikely until after he hands over to President-elect Raisi early next month.

Raisi is an ultraconservative but has expressed support for the nuclear talks, arguing Iran needs an end to US sanctions.

Iran's ultraconservative camp, which deeply distrusts the United States, has repeatedly criticised Rouhani for the 2015 deal.