US President Joe Biden said on Friday that he believed Iran was serious about negotiations over its nuclear programme, but it was unclear how serious.
When asked if he thought Tehran was serious about indirect talks between the US and Iran that have resumed in Vienna, Mr Biden replied, "Yes, but how serious, and what they are prepared to do is a different story. But we're still talking."
Efforts to revive the Iran nuclear deal are "showing movement", Iranian deputy foreign minister Sayed Kazem Sajjadpour said, as talks continued in Vienna.
Mr Sajjadpour said negotiations had moved from "debating what to do" towards a drafting stage as diplomats try to bring the US and Iran back in line with the deal.
But, he said, there were still “psychological and political barriers” to concluding a deal.
The talks in Vienna resumed on Friday, with Iran and the US negotiating indirectly via European diplomats.
Britain, France, Germany, Russia and China are also involved in the talks aimed at reviving the deal the US abandoned under former president Donald Trump in 2018.
Russia’s delegate in Vienna, Mikhail Ulyanov, voiced optimism over Friday’s talks and suggested they could be the final round of negotiations.
"The participants agreed on the need to intensify the process," he said.
"The delegations seem to be ready to stay in Vienna as long as necessary to achieve the goal."
US Secretary of State Antony Blinken sounded a more cautious note this week, saying that it was still unclear whether Iran was willing to strike a deal.
“We’ve been engaged in Vienna for some weeks with our European partners, Russia and China, and indirectly with Iran,” Mr Blinken told BBC Radio 4.
“We’ve demonstrated our seriousness of purpose in terms of wanting to get back into the so-called JCPOA [Joint Comprehensive Plan of Action].
"What we don’t yet know is whether Iran is prepared to make the same decision and move forward.”
Iran is demanding that all US sanctions be lifted before it will resume respecting the limits on its nuclear activity that it agreed to in 2015.
Mr Sajjadpour, who heads a research institute in Iran’s foreign ministry, said that the talks were “showing movement to a better position”.
He told a panel on the fringes of the European University Institute’s State of the Union event that there were three stages to the negotiations.
“One is, let’s say, debating what to do,” he said. “I think it took a long time but it worked, so the debate is over.
“The second stage was drafting, and when you enter the drafting period it means that debate has moved. This drafting stage needs to be polished but I think it's going on.
“However, there is a further stage, and that is the decision to implement. I think this is where there are psychological barriers, there are political barriers.
“But it depends on the political will of all sides, but I think mostly the United States.”
Summing up the state of the talks, Mr Sajjadpour said: “I am not pessimistic, although optimism requires more work to be done.”
Iran's nuclear ambitions in the spotlight
The nuclear limits under the deal were intended to prevent Iran from acquiring a nuclear weapon, something Tehran insists it has no intention of doing.
After the US quit the deal and restored sanctions, Iran breached limits on its nuclear activity that were intended to prevent it from acquiring a nuclear bomb.
Last month, Iran began enriching uranium to 60 per cent, far above the 3.67 per cent cap under the deal.
Iran denies seeking a nuclear weapon and insists its nuclear activity is for civilian purposes.
However, recent reports by German and Dutch intelligence named Iran among a series of countries seeking to source materials from Europe to develop weapons of mass destruction.
Western powers also face pressure to link a deal with Iran to the release of foreign citizens detained in the Islamic republic.
Among them is British mother Nazanin Zaghari-Ratcliffe, whose family believes that a UK debt of about $550 million owed to Iran is a major reason for her detention.
The UK denied a report on Iranian television over the weekend that suggested the British government had paid the debt.
Asked whether the US would oppose a UK decision to pay the debt, Mr Blinken said: “It’s a sovereign decision for the UK.”