US Secretary of State John Kerry joined troubled Iran nuclear talks in Vienna on Thursday, four days before a looming deadline, with Russia warning that getting a deal will be "very difficult".
Speaking in Paris earlier, Kerry said that with British counterpart Philip Hammond -- who said on Wednesday he was "not optimistic" -- and the other powers he was "concerned about the gaps".
"We all are," Kerry said.
Hammond also suggested that the best hope was making enough progress to extend the deadline for a second time after an earlier cut-off point of July 20 was missed.
But Kerry, who went into a late-night meeting with Iranian Foreign Minister Mohammad Javad Zarif soon after arriving, said the parties "are not discussing an extension. We are negotiating to have an agreement."
Iran and the six powers have been negotiating intensively since February to turn an interim accord with Iran reached a year ago into a lasting agreement before November 24.
Such a deal, after 12 years of rising tensions, is aimed at easing fears that Tehran will develop nuclear weapons under the guise of its civilian activities -- an ambition the Islamic republic has always hotly denied.
- 'Tense atmosphere' -
Russia's main negotiator in the talks, Sergei Ryabkov, said Thursday that the talks were being held in a "tense atmosphere" and that agreeing the mammoth accord would be tough.
"In the current situation it will be very difficult to get a deal unless there is a new spirit," Ryabkov was quoted as saying by Russian agency RIA Novosti.
He warned: "A possibility like we have at the moment (to get a deal) is very rare. This is a crucial moment and to let it pass would be a serious mistake with grave consequences."
Some areas appear provisionally settled in what would be a highly complex deal that would run for many years, even decades.
But two key issues remain: enrichment -- rendering uranium suitable for peaceful uses but also, at high purities, for a weapon -- and the pace of the lifting of sanctions.
Diplomats say Iran wants all sanctions lifted at once. The six world powers want however to stagger any suspension to be sure that Iran would not renege on its commitments.
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Iran wants to massively ramp up the number of enrichment centrifuges -- in order, it says, to make fuel for a fleet of future reactors -- while the West wants them dramatically reduced.
Iran's nuclear chief Ali Akbar Salehi stuck to this position on Thursday, saying Iran would increase its enrichment capacity to 190,000 SWU (Separative Work Units) -- around 20 times its current ability -- within eight years.
The six powers say Iran has no such need in the foreseeable future. Russia is contracted until 2021 to fuel Iran's only power reactor at Bushehr and last week signed a deal to build -- and fuel -- several others.
Salehi also said the much-reported idea of exporting its stockpile of low-enriched uranium -- enough for around eight bombs if purified to weapons grade -- "makes no sense".
- Upping the ante -
Upping the ante, Iran's President Hassan Rouhani said on Wednesday that a deal was possible but only if the other side showed "political will ... and doesn't make excessive demands".
Iran's speaker of parliament Ali Larijani meanwhile told Iranian media: "We are constantly cooperating (but the other side) is raising the tone."
He added: "We hope that the other side will behave in a rational manner ... and won't take the wrong path."
France said Foreign Minister Laurent Fabius would be in Vienna on Friday but it was unclear when other counterparts might arrive.
Russian Foreign Minister Sergei Lavrov, whose country is a crucial player in the talks, will only attend if there is sufficient progress, Ryabkov told Russian media this week.
"With the arrival of Kerry the talks will move into a more serious direction," RIA Novosti cited a source in the Russian delegation as saying on Thursday.
"Much depends on what decisions he will bring from Washington".
Lavrov was due to meet his Saudi counterpart in Moscow on Friday, and therefore cannot be in Vienna before Saturday, a foreign ministry spokesman said in Moscow on Thursday.
"A deal is still possible by November 24," Arms Control Association analyst Kelsey Davenport told AFP.
"The remaining obstacles can be overcome if both sides are willing to show some flexibility."