The deal, which curbs Iran's nuclear programme in exchange for sanctions relief, has been on life support since then US president Donald Trump bolted in 2018.

The remaining partners to the 2015 accord have been engaged in negotiations since early April to try to revive it.

The third round of talks started on Tuesday and, after several days of technical discussions between expert groups and delegations, will resume on Saturday.

The service of EU foreign policy chief Josep Borrell said in a statement that delegates from Britain, China, France, Germany, Iran and Russia would meet in person at around 3 pm (1300 GMT).

"Participants will continue their discussions in view of a possible return of the United States to the JCPOA and on how to ensure the full and effective implementation of the JCPOA," the statement said, using the acronym of the agreement's formal name.

After the talks, the delegations will return to their respective capitals to receive instructions, Iran's foreign ministry said.

A European diplomat said that the American, European, Russian and Chinese delegations held a joint meeting on Saturday morning, but without Iranian representation as Tehran has refused to negotiate with the US directly.

As well as bolting from the accord, Trump's administration slapped sweeping sanctions on Iran, which in turn started ramping up its nuclear activities.

New US President Joe Biden supports the agreement -- with which Iran was in compliance before Trump's sanctions -- but has called on Tehran to roll back its measures before Washington ends sanctions.

After almost a month of talks, the parties this week expressed a desire to "speed up" the negotiations.

The hope of the talks is to achieve a concrete result "by the end of May" -- before Iran's presidential election in June -- a diplomat familiar with the discussions told AFP during the last round.