Iran has been engaged in direct talks in Vienna to revive the accord, known formally as the Joint Comprehensive Plan of Action (JCPOA), with France, Germany, Britain, Russia and China directly. The United States, which unliterally pulled out of the deal in 2018, is participating indirectly.

"We had four issues as our red lines," but "two issues have almost been resolved," Hossein Amir-Abdollahian was quoted as saying Wednesday by state news agency IRNA, adding that "two issues remain, including (an) economic guarantee".

He did not elaborate on the second issue.

"If the American side fulfils our two remaining demands today, we will be ready to go to Vienna tomorrow," he said.

More than 10 months of negotiations had brought the parties close to renewing the landmark accord.

But the talks were halted after Russia this month demanded guarantees that Western sanctions imposed following its invasion of Ukraine would not damage its trade with Iran.

Amir-Abdollahian's remarks come a day after his visit to Moscow, where counterpart Sergei Lavrov said Russia had received the necessary guarantees from Washington on trade with Iran.

The 2015 agreement gave Iran sanctions relief in exchange for curbs on its nuclear programme to guarantee that Tehran could not develop a nuclear weapon -- something it has always denied wanting to do.

But the US's unilateral withdrawal from the accord in 2018 under then-president Donald Trump and the reimposition of biting economic sanctions prompted Iran to begin rolling back on its own commitments.

- Agreement 'on the table' -

During the talks in Vienna, Iran has repeatedly called for guarantees from the US administration of President Joe Biden that there will be no repeat of Trump's pull-out.

Iranian and US delegations in Vienna do not communicate directly, but messages are passed through other participants and the European Union, the talks' coordinator.

"We will continue to exchange our messages to the American side... through (the EU's) Enrique Mora," Iran's top diplomat said.

The Vienna talks aim to return the US to the nuclear deal, including through the lifting of sanctions on Iran, and to ensure Tehran's full compliance with its commitments.

France on Wednesday said that concluding the talks was "urgent".

"We have taken note of Russia's position. Once again, we call on all parties to adopt a responsible approach and to make the necessary decisions in order to finalise the just and comprehensive agreement that is on the table," a foreign ministry spokeswoman said.

"It is critically urgent that a conclusion be reached," she added.

The comments come the same day as two British-Iranians flew home after being released from years of detention in Iran, and as the UK government confirmed it had paid a long-standing debt over a cancelled defence contract.

Nazanin Zaghari-Ratcliffe and Anoosheh Ashoori "will be reunited with their families and loved ones", the UK's Foreign Secretary Liz Truss said Wednesday.

She also announced that Morad Tahbaz, an Iranian-American who also holds British nationality, has been released from prison "on furlough" to his Tehran home.

During the nuclear talks in Vienna, Iranian and American officials have signalled that the two sides are indirectly discussing a possible prisoner exchange.

The two sides have made prisoner swaps in the past and are currently known to hold four of each other's nationals.