Russian Foreign Minister Sergei Lavrov was rushing back to the Swiss city of Lausanne to "take part in the final part of the ministerial meeting of the six powers."
"Chances are high," Lavrov told a press conference in Moscow.
"The prospects of this round of talks are not bad, even good I would say," he added.
An army of technical and sanctions experts had worked into the early hours today, exchanging documents to form the basis of an accord.
They would have until June 30 to flesh it out with annexes.
US Secretary of State John Kerry, in Lausanne since Wednesday in the latest in a series of meetings around the world with his Iranian counterpart Mohammad Javad Zarif, said late Monday there "still remain some difficult issues".
A meeting between Kerry and his counterparts from the other five powers began shortly after 7:00 am (0500 GMT).
They were joined later by Zarif and other members of the Iranian nuclear team, including nuclear chief Ali Akbar Salehi, for a full session which lasted about 40 minutes.
But the negotiations still hung in the balance today, as uncertainty reigned whether a deal could be reached by midnight (2200 GMT).
The stakes are high, with fears that failure to reach a deal may set the United States on a road to military action to thwart Iran's nuclear drive.
Israel has also upped the pressure against what it calls a "dangerous" deal, which it fears could embolden Iranian regional ambitions.
Under the deal, the powers want Iran to scale back its nuclear programme to give the world ample notice of any dash to make the bomb by extending the so-called "breakout" time.
In return, the Islamic republic, which denies wanting atomic weapons, is demanding the lifting of sanctions that have strangled its economy.
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