EU deputy secretary general Enrique Mora tweeted earlier that “intense preparatory work was ongoing”, adding that he had already met with Russia and China ahead of further discussions with European and US officials.
The JCPOA deal has stalled since 2018, when ex-US president Donald Trump pulled out of the agreement.
Although the remaining signatories to the deal have tried to keep the accord alive, talks have been on hold for the past five months following the election of new Iranian president Ebrahim Raisi, a hard-line cleric.
Diplomats were however playing down hopes of a breakthrough, with Iran demanding a lifting of sanctions in return for them halting their nuclear programmes and improving cooperation with the international nuclear watchdog the IAEA.
Ahead of the talks, UK Foreign Secretary Liz Truss said she will “work night and day” with Israel to halt Iran’s nuclear ambitions.