"One can say with relative certainty that we at the minister level have reached an agreement in principle on all key aspects of the final settlement of this issue," Russian media quoted Sergei Lavrov as saying at talks in Switzerland.
This came after Russia's top diplomat and the foreign ministers of five other major powers and Iran continued working through the night as they missed a midnight deadline to agree the main outlines of what they hope will be an historic accord.
The stakes are high, with fears that failure to reach a deal may set the United States and Israel on a road to military action to thwart Iran's nuclear drive, which Tehran says is purely peaceful.
With talks set to resume this morning, confusion remained about the exact status of the negotiations.
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The "agreement in principle... Will be put on paper in the coming hours or perhaps within one day," Lavrov said, quoted by Ria Novosti after a lengthy day of talks in Lausanne.
"We have accomplished quite a bit, but people needed to get some rest and start over early in the morning. I hope that we can finalise the work on Wednesday," Zarif told reporters.
A senior US official however said there was not yet full agreement on key points.
"All issues have not been agreed," a senior US official told AFP.
US Secretary of State John Kerry briefed President Barack Obama, and his national security team on the day's negotiations by secure video conference.
A Western diplomat also said there was no framework agreement yet.