Despite huge gaps in the negotiations, "we have never been closer in more than 10 years," Steinmeier said as he joined the talks in Vienna.
"If Iran is ready to take this opportunity then movement is possible ... Whether we can get a result is right now completely open," he said.
"We will do whatever we can and if we can't then we will leave ourselves open to the accusation that we have missed out on something that could have resolved this standoff," Steinmeier added.
At stake in the Austrian capital Vienna is a historic deal in which Iran would curb its nuclear activities in exchange for broad relief from years of heavy international economic sanctions.
It could end a 12-year standoff with the West that has even raised the threat of Israeli military strikes against Iranian nuclear facilities.
Steinmeier was due to be briefed by US Secretary of State John Kerry, who has met with his Iranian counterpart Mohammed Javad Zarif here three times since Thursday.
Iranian sources said Zarif was also due to meet the German top diplomat.
Besides Iran, Germany and the United States, the talks involve Russia, China, France and Britain.