"Regarding the negotiations in Vienna, they aren't easy -- that's been clear in recent weeks," Maas told a joint press conference with US Secretary of State Antony Blinken.

"Nevertheless we are moving forward step by step in every round of negotiations and we assume that in the context of the (Iranian) presidential election, there's a good chance to conclude them in the foreseeable future."

Maas said there were still "several technical questions to resolve" on the landmark accord.

"In any case we are convinced that it's worth it to maintain talks over such a long period of time in order to revive the JCPOA in this way."

The 2015 deal known as the Joint Comprehensive Plan of Action (JCPOA) saw Iran accept curbs on its nuclear activities in return for an easing of sanctions.

But in 2018 then US president Donald Trump unilaterally abandoned the agreement and ramped up sanctions, prompting Iran to pull back from its own commitments.

Trump's successor Joe Biden has signalled his readiness to return to the deal and state parties -- also including Britain, China, France, Germany and Russia -- have been negotiating its revival in Vienna since early April.

Ultraconservative cleric Ebrahim Raisi scored 62 percent of the vote in a presidential election in the Islamic republic last Friday.

Raisi, seen as close to supreme leader Ayatollah Ali Khamenei, who holds ultimate political power in Iran, said on Monday he would not allow the nuclear negotiations to drag on.

EU negotiator Enrique Mora said on Sunday that those involved in the talks were "closer" to saving the Iran nuclear deal but that sticking points remain.