Fresh talks on reviving the 2015 nuclear deal between Iran and world powers restarted on Monday, with the EU chair saying he felt "extremely positive" while admitting that "difficult issues" have yet to be tackled.
The talks in Vienna are the first since Iran paused them in June after the election of ultraconservative new President Ebrahim Raisi. Diplomats at the time had said they were "close" to an agreement.
Iran ignored appeals from Western countries to restart the talks for several months, all the while strengthening the capabilities of its nuclear programme. In August, Raisi said Iran was again open to talks.
Monday's meeting started just after 3 pm (1400 GMT) in the Palais Coburg hotel where the 2015 agreement -- known as the Joint Comprehensive Plan of Action (JCPOA) -- was clinched, and lasted a little more than two hours.
Along with Iran, diplomats from Britain, China, France, Germany and Russia are attending.
Enrique Mora, the EU official chairing the talks, said there was "a sense of urgency in bringing the JCPOA back to life" and added that he felt "extremely positive".
However, he admitted that "there are still difficult issues ahead".
The United States is taking part in the talks indirectly and has said that Iran's recent actions do not "augur well" for the prospects of reviving the deal.
Iran's foreign ministry said in a statement that Monday's meeting had taken place in a "professional and serious atmosphere".
The head of Iran's delegation Ali Bagheri had "underlined the necessity to make sanctions lifting an absolute priority for the talks", it said.
The JCPOA offered a lifting of some of the array of economic sanctions Iran had been under in return for strict curbs on its nuclear programme.
But the deal started to unravel in 2018 when then-US president Donald Trump pulled out and began reinstating sanctions on Iran.
Ordinary Iranians are hoping the talks may lead to some of those crippling sanctions being lifted.
Unemployed Tehran resident Davoud Lotfinia told AFP: "The sanctions probably haven't affected the authorities, but the purchasing power of ordinary people is diminishing every day."
The EU's Mora said that there was an "urgency in putting an end to the suffering of the Iranian people".
The year after Trump's move, Iran retaliated by starting to exceed the limits on its nuclear activity laid down in the deal.
In recent months, it has started enriching uranium to unprecedented levels and has also restricted the activities of inspectors from the International Atomic Energy Agency (IAEA), the UN watchdog charged with monitoring Iran's nuclear facilities.
IAEA Director General Rafael Grossi said "no progress" was made on issues he raised during a visit to Tehran last week.
Mora said on Monday that getting Iran's nuclear programme under "transparent monitoring" was a matter of "urgency".
A working group on lifting sanctions would meet on Tuesday, Mora said, with a group focusing on nuclear-related commitments meeting the following day.
"Iran is acting like the United States is going to blink first but... pressure is a double-edged sword," Kelsey Davenport, an expert with the Arms Control Association, told journalists last week.
"If there are gaps in the IAEA's monitoring, it will drive the speculation that Iran has engaged in illicit activity, that it has a covert programme, whether there's evidence to that or not," Davenport said, which could in turn "undermine the prospects for sustaining the deal".
In London, top Israel diplomat Yair Lapid was scheduled to meet British Prime Minister Boris Johnson on Monday, and French President Emmanuel Macron in Paris on Tuesday.
Lapid also met his British counterpart Liz Truss, and before the meeting the pair published an article in the Daily Telgraph newspaper saying they would "work night and day to prevent the Iranian regime from ever becoming a nuclear power".
British foreign minister Liz Truss added in a statement that the UK wanted "Iran to agree to the original JCPOA" but warned that if the talks "don't work, all options are on the table".
Israeli Prime Minister Naftali Bennett urged the country's allies "to not give into Iran's nuclear blackmail", adding: "Such a murderous regime should not be rewarded."