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