Iran's nuclear talks are due to enter the make-or-break endgame, days ahead of a deadline that may not be met as few, but significant, hurdles stand in the way. 

Officials from Iran and six world powers - US, China, Russia, Britain, France and Germany - arrived in Austria's capital Vienna for the final round of talks starting on Friday, in which it is hoped Iran will make concessions on its nuclear programme in exchange for a lifting of sanctions. The deal has to be reached before Monday's deadline.

Speaking in Paris on Thursday before flying to Vienna, US Secretary of State John Kerry said: "We are not discussing an extension. We are negotiating to have an agreement. It's that simple."

He added, however, that the US and all the other powers were "concerned about the gaps".

Britain's Foreign Secretary Philip Hammond had said on Wednesday that he was "not optimistic" the deadline could be met.

"I think if we make some significant movement, we may be able to find a way of extending the deadline to allow us to get to the final deal," Hammond said in Latvia's capital Riga. 

Ghanbar Naderi, an Iranian journalist in Tehran, told Al Jazeera that he would not be surprised if there was another extention of the talks because neither side was willing to make significant concessions. 

Russia's main negotiator in the talks, Sergei Ryabkov, was quoted by Russian news agency RIA Novosti on Thursday as saying "tense atmosphere" shadows the talk, and that "in the current situation it will be very difficult to get a deal unless there is a new spirit".

French Foreign Minister Laurent Fabius, who together with his British counterpart was expected in Vienna on Friday, said at a joint news conference with Kerry in Paris that he hoped differences could "be bridged but that depends to a very large extent on Iran's attitude".

'Raising the tone'

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."

Iran's Foreign Minister Mohammad Javad Zarif believes that the onus is on the six powers, urging them not to make "excessive demands".

Iran and the six powers have been negotiating intensively since February to turn an interim accord 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 civilian use, claims which Islamic republic repeatedly denies.

Two key issues remain in a highly complex deal that may run for decades: enrichment of uranium, and the pace of the lifting of sanctions.

While Iran reportedly wants all sanctions lifted at once, the six world powers want a phased suspension to be sure that Iran would not renege on its commitments.

Furthermore, 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 around 20 times its current ability within eight years.

Source: Al Jazeera and agencies