US Secretary of State John Kerry has warned of "serious gaps" in talks about a nuclear deal with Iran.
But as Monday's deadline approached his German counterpart said Tehran and six world powers have "never been closer" to agreement since they started negotiating more than six years ago.
German foreign minister Frank-Walter Steinmeier also said that both sides are "still far apart" on certain questions. But he suggested the differences are bridgeable, declaring that the talks have reached "a moment of truth." Still, he said, success or failure "is still completely open at this point".
Mr Steinmeier spoke after arriving in Vienna to join Mr Kerry's efforts to move the talks forward and shortly before meeting with the chief US diplomat.
High-level comings and goings since Friday also have seen Foreign Secretary Philip Hammond and French foreign minister Laurent Fabius stop by for talks with Mr Kerry, Iranian foreign minister Mohammad Javad Zarif and other participants in the negotiations.
"We're working hard," Mr Kerry said on Saturday. "But we... still have some serious gaps."
Mr Kerry spoke by telephone on Saturday to Arab foreign ministers in the Gulf, whose countries fear Iran's potential abilities to make nuclear arms, and with his Canadian and Turkish counterparts, the US State Department said.
Hopes of progress were briefly boosted on Friday, with reports that Mr Zarif planned to fly to Tehran for additional consultations. That could have meant possible progress, suggesting that the Iranians need political approval from Tehran to move forward.
Iranian media initially spoke of a new US initiative that Mr Zarif needed to have his superiors approve, but the Iranian diplomat dashed those hopes, saying he was staying in Vienna and had "no remarkable offers and ideas to take to Tehran."
The United States - backed by Russia, China, Britain, France and Germany - is seeking a deal that cuts, and puts long-term limits on, Iranian nuclear programmes that could be used to make weapons. Iran says it does not want such arms but is negotiating in the hope of relief from sanctions imposed because of its nuclear activities.
Mr Kerry and Mr Zarif have both emphasised that there has been no discussion about extending the talks, if the deadline is not met. However, big differences in the negotiations increasingly suggest that both sides could agree to continue talking past Monday.