The United States and its partners are likely to exert pressure on Tehran if it uses talks scheduled to resume on Monday as a pretext to accelerate its nuclear programme, the US special envoy for Iran has said.
Robert Malley is leading the US negotiating team in the indirect talks with Iran in Vienna. The negotiations, with the participation of major powers, resume after a five-month hiatus. They will be the first to be held since Iran's hardline President Ebrahim Raisi took office in August.
"If Iran thinks it can use this time to build more leverage and then come back and say they want something better, it simply won't work. We and our partners won't go for it," Mr Malley told the BBC in an interview broadcast on Saturday.
Tehran is demanding the immediate lifting of all sanctions imposed by former US president Donald Trump after he pulled Washington out of the 2015 nuclear agreement.
The pact, officially called the Joint Comprehensive Plan of Action, reached between Iran and the world powers in 2015, lifted restrictions on Tehran in return for curbs on its atomic activities.
Iran responded to the US withdrawal from the deal in 2018 by increasing its levels of uranium enrichment and other nuclear activities.
"If that's Iran's approach, which is to try to use the negotiations as cover for an accelerated nuclear programme and, as I say, drag its feet at the nuclear table, we will have to respond in a way that is not our preference," Mr Malley said.
"Nobody should be surprised if at that point there is increased pressure on Iran.
"We hope not to get that there but if we are, then pressure will have to increase to send a message to Iran that the choice it is making is the wrong one – that it has a different path available to it but it's not a path open indefinitely because Iran's nuclear programme is putting the very essence of the deal negotiated [in 2015] at risk."
On Sunday, Iran's negotiating team, led by Ali Bagheri Kani, held bilateral and trilateral meetings in Vienna, the semi-official ISNA news agency reported, before a resumption of the nuclear talks designed to revive the agreement.
"The Iranian team arrived on Saturday in Vienna and started meetings, which continued on Sunday at an expert level with the heads of the Russian and Chinese negotiating teams, as well as the EU Co-ordinator, Enrique Mora," Iranian diplomat Mohammadreza Ghaebi told ISNA.
Israeli Prime minister Naftali Bennett said his country is "distrurbed" by the willingness of world powers to lift sanctions on Iran.
"Israel is very disturbed by the willingness to lift sanctions and allow billions (in US dollars) to flow into Iran in exchange for insufficient nuclear restrictions," he said during a cabinet meeting.
This is the message that we convey in every way to the Americans and to all the countries negotiating wit Iran. The foreign minister will convey a similar message in his meetings in London and Paris this week,” Mr Bennett added.
In the meantime, diplomats say time is running out to resurrect the pact.
Six rounds of indirect talks were held between April and June. The new session begins after a hiatus triggered by the election of Mr Raisi.
"They are doing enough technically so they can change their basic relationship with the West to be able to have a more equal dialogue in the future," said a Western diplomat involved in the talks.
Two European diplomats said it seemed that Iran was simply playing for time to accumulate more material and know-how.
Iran's leading negotiator and its foreign minister both repeated on Friday that full lifting of sanctions would be the only thing on the table in Vienna.
"If this is the position that Iran continues to hold on Monday, then I don't see a negotiated solution," one of the European diplomats said.
Several diplomats said Iran was now four to six weeks away from the "breakout time" it needs to amass enough fissile material for a single nuclear weapon but it was still about two years from being able to use it.
Should the talks collapse, the likelihood is the US and its allies will initially confront Iran at the International Atomic Energy Agency gathering next month by calling for an emergency meeting.