German Foreign Minister Heiko Maas said Friday that Iran was "delaying" talks to salvage the 2015 nuclear deal, warning that the option to revive the deal would not remain open forever.

"I see with growing unease that Iran is on the one hand delaying the resumption of nuclear negotiations in Vienna, and on the other moving ever further away from the core elements of the deal," Maas said in an interview with Der Spiegel weekly.

Iran resumed talks in April with world powers to revive the 2015 nuclear deal that gave the country some relief from international sanctions in exchange for limits on its nuclear programme.

The deal, formally known as the Joint Comprehensive Plan of Action (JCPOA), was torpedoed in 2018 when then US president Donald Trump unilaterally withdrew from it and reimposed sanctions.

Negotiations in Vienna, in which the US is indirectly involved, have stalled as Iran awaits the inauguration of new ultra-conservative president Ebrahim Raissi at the beginning of August. 

"We want a return to the JCPOA, and we are firmly convinced that it is in everyone's interest," said Maas.

"What is clear, however, is that this option will not be open to us forever," he added.

US Secretary of State Antony Blinken also said Thursday that talks with Iran could not go on indefinitely. 

"The ball remains in Iran's court," he said, adding that the US was "fully prepared to return to Vienna to continue negotiations".