US Secretary of State John Kerry has said that extending the deadline for a deal on Iran’s nuclear programme is not being discussed.
Negotiations in Vienna will instead focus on “driving towards” the outline of an agreement, he said.
World powers want Iran to show it is not building nuclear weapons, in exchange for a lifting of sanctions.
Diplomats are bracing for tough negotiation at the six-nation talks, with a Monday deadline for a deal.
“In the current situation it will be very difficult to get a deal unless there is a new spirit,” Russian chief negotiator Sergei Ryabkov told Russian media. “The talks are being held in a tense atmosphere.”
The US, EU and other powers suspect Iran is secretly seeking to develop nuclear weapons.
Tehran denies its nuclear programme, which is being developed in part with Russian technology, has a military aspect, insisting it is only intended to supply its energy and medical needs.
