Mr Johnson is meeting with German Chancellor Angela Merkel, US President Joe Biden and French President Emmanuel Macron, known as the "Big Four leaders", to discuss the issue.

       President Biden is trying to revive the 2015 nuclear deal and bring Iran back into compliance with the pact that would have kept the Islamic republic at least one year away from the potential to field a nuclear weapon.

       The UN’s atomic watchdog has said Iran is increasingly in violation of the deal, known as the Joint Comprehensive Plan of Action.

       President Donald Trump withdrew the US from the 2015 nuclear deal but Washington has participated indirectly in talks aimed at bringing both Washington and Tehran back into compliance.

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       The Vienna talks have been on hiatus since June when Iranian President Ebrahim Raisi took power.

       But Ali Bagheri, Iran's deputy foreign minister and chief negotiator for the talks, tweeted that Iran has agreed to restart negotiations by the end of November and a date for a resumption of talks "would be announced in the course of the next week."

       A senior US administration official told reporters at the Summit that the meeting was Mrs Merkel's initiative and would give the leaders an opportunity to go over the topic ahead of a critical period coming up.

       Meanwhile, US National Security Advisor Jake Sullivan said the US was still trying to determine whether Iran was serious about the negotiations.

       Downing Street added the “Iran nuclear deal was the best option on the table for preventing Iran from getting nuclear weapons.”

       A spokesperson added: "It is extremely important that Iran returns to compliance.”

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       The crunch meeting comes as Washington officials blamed Iran for a drone attack on a remote US outpost in Syria.

       Washington said on Monday that the US believes Iran resourced and encouraged the attack, but that the drones were not launched from Iran.

       No deaths or injuries were reported as a result of the attack.

       In retaliation, the US Treasury Department on Friday announced new penalties against two senior members of Iran's Revolutionary Guard Corps and two affiliated companies for supplying lethal drones and related material to insurgent groups in Iraq, Lebanon, Yemen and Ethiopia.

       The meeting also comes after Mohammad Eslami, head of Iran’s atomic energy organisation, said on national television in October: "We have passed 120 kilograms.

       "We have more than that figure.

       "Our people know well that [Western powers] were meant to give us the enriched fuel at 20 percent to use in the Tehran reactor, but they haven’t done so.

       "If our colleagues do not do it, we would naturally have problems with the lack of fuel for the Tehran reactor."