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.
An EU statement on Thursday voiced "regret" that Iran had yet to address UN nuclear inspectors' concerns. Mr Kerry arrived in Vienna on Thursday evening and met Iranian Foreign Minister Mohammad Javad Zarif.
Speaking earlier in Paris, Mr Kerry said he was "quite confident" with the groundwork laid for a possible deal.
He added: "We are not talking about an extension, not among ourselves." Among those less confident about a deal being reached by the self-imposed deadline of 24 November was an unnamed Iranian official who told Reuters news agency that more time was needed to "resolve technical issues" and that the time frame for lifting sanctions was "still a huge dispute".
A Western official told Reuters an "interim agreement" was more likely or perhaps "at best a framework agreement by Monday that needs to be worked out in the coming weeks and months".
Speaking in Moscow, Mr Ryabkov said it was a "crucial moment" for achieving a deal and to let it pass would be a "serious mistake with grave consequences".
The six nations involved in the talks with Iran are the US, the UK, France, Germany, Russia and China.
It is unclear what, if any, effect the recent Western sanctions on Russia over Ukraine may have on Moscow's position at the talks, amid signs Russia and Iran want to expand economic ties.
Diplomats quoted by Reuters news agency say the West is willing to compromise but Iran is not, largely because Supreme Leader Ayatollah Ali Khamenei has not given the negotiators the freedom to do so.
"The ball is in the Iranian camp and to be honest we have a feeling that we're treading water at the moment," the agency quoted an unnamed senior Western diplomat as saying. "The main obstacle is that the decisions have to be made by the Iranian leadership." Brussels expressed its concern in Vienna at a meeting of the International Atomic Energy Agency (IAEA), at which the nuclear watchdog again accused Iran of failing to address doubts over its nuclear work.
"The EU deeply regrets the lack of progress on PMD [possible military dimensions] issues," the EU statement said.
"The EU underlines that resolving all outstanding issues will be essential to achieve a comprehensive, negotiated long-term settlement, which is the EU's objective."
EU foreign affairs chief Federica Mogherini urged Iran to take a "strategic decision" to achieve a landmark deal.
IAEA chief Yukiya Amano said earlier Iran had not "provided any explanations that enable the agency to clarify the outstanding practical measures".
Iran had promised to give the IAEA a response by late August to allegations concerning explosives tests and other activity that could be linked to a nuclear weapons programme.
The deal being sought by the world powers is not directly linked to the IAEA's investigation of Iran.
However, the US has said in the past that Iran must address the watchdog's concerns if it expects a comprehensive agreement in the talks.
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.
An EU statement on Thursday voiced "regret" that Iran had yet to address UN nuclear inspectors' concerns. Mr Kerry arrived in Vienna on Thursday evening and met Iranian Foreign Minister Mohammad Javad Zarif.
Speaking earlier in Paris, Mr Kerry said he was "quite confident" with the groundwork laid for a possible deal.
He added: "We are not talking about an extension, not among ourselves." Among those less confident about a deal being reached by the self-imposed deadline of 24 November was an unnamed Iranian official who told Reuters news agency that more time was needed to "resolve technical issues" and that the time frame for lifting sanctions was "still a huge dispute".
A Western official told Reuters an "interim agreement" was more likely or perhaps "at best a framework agreement by Monday that needs to be worked out in the coming weeks and months".
Speaking in Moscow, Mr Ryabkov said it was a "crucial moment" for achieving a deal and to let it pass would be a "serious mistake with grave consequences".
The six nations involved in the talks with Iran are the US, the UK, France, Germany, Russia and China.
It is unclear what, if any, effect the recent Western sanctions on Russia over Ukraine may have on Moscow's position at the talks, amid signs Russia and Iran want to expand economic ties.
Diplomats quoted by Reuters news agency say the West is willing to compromise but Iran is not, largely because Supreme Leader Ayatollah Ali Khamenei has not given the negotiators the freedom to do so.
"The ball is in the Iranian camp and to be honest we have a feeling that we're treading water at the moment," the agency quoted an unnamed senior Western diplomat as saying. "The main obstacle is that the decisions have to be made by the Iranian leadership." Brussels expressed its concern in Vienna at a meeting of the International Atomic Energy Agency (IAEA), at which the nuclear watchdog again accused Iran of failing to address doubts over its nuclear work.
"The EU deeply regrets the lack of progress on PMD [possible military dimensions] issues," the EU statement said.
"The EU underlines that resolving all outstanding issues will be essential to achieve a comprehensive, negotiated long-term settlement, which is the EU's objective."
EU foreign affairs chief Federica Mogherini urged Iran to take a "strategic decision" to achieve a landmark deal.
IAEA chief Yukiya Amano said earlier Iran had not "provided any explanations that enable the agency to clarify the outstanding practical measures".
Iran had promised to give the IAEA a response by late August to allegations concerning explosives tests and other activity that could be linked to a nuclear weapons programme.
The deal being sought by the world powers is not directly linked to the IAEA's investigation of Iran.
However, the US has said in the past that Iran must address the watchdog's concerns if it expects a comprehensive agreement in the talks.