"We must now use the newly opened time window, we must leave nothing undone to reach the solution that has eluded us in recent years," Frank-Walter Steinmeier said before the talks with Iranian Foreign Minister Mohammad Javad Zarif.
In a brief joint press appearance with Zarif, Steinmeier said "we probably share the understanding that this is now the decisive phase of the negotiations".
Iran and major world powers have given themselves until late June to reach a comprehensive agreement that would prevent Tehran from developing a nuclear bomb, a goal it denies having, in return for an easing of punishing economic sanctions.
Sunday will see talks in Geneva between Iran and the so-called P5+1 group - the United States, Britain, China, France, Germany and Russia - seeking to break a stalemate that has seen two earlier deadlines pass without an accord.
Steinmeier said "we have extended this transition agreement twice but we also agreed at the last meeting that we share the common understanding that one cannot indefinitely continue the extensions".
"Iran's path to nuclear weapons must end unambiguously, verifiably and permanently, and in return sanctions must be lifted credibly and step-by-step," Steinmeier added.
He added that this would restore trust between all sides as they faced a host of crises and conflicts such as the threat posed by the Islamic State jihadist group in Iraq and Syria.
'11 years lost'
"We have lost 11 years, and the conditions have not become better. And that's why we need to seize the opportunity to achieve justice, peace and security, and I'm certain that with the participation of Germany... we can reach this goal," Zarif said.
Later Thursday, Zarif met European Union foreign affairs head Federica Mogherini who insisted that the "negotiations have to be brought to a conclusion in line with the agreed time."
Mogherini also discussed a "wide range" of topics including the crisis in Syria and Iraq.
Mogherini "encouraged Iran to use its considerable influence to help create a more inclusive and stable Iraq, which is in both the EU's and Iran's interests," a statement said.
Zarif met US Secretary of State John Kerry in Geneva on Wednesday and is due to meet French Foreign Minister Laurent Fabius in Paris on Friday.