World powers and Iran began discussing late Sunday whether more time is needed to reach a nuclear deal, a US official said, as they struggled to overcome major gaps barely 24 hours before a deadline.

The five permanent members of the UN Security Council and Germany (the P5+1) have been locked in talks with Iran for months to turn an interim deal struck in Geneva that expires on Monday into a lasting accord.

Such an agreement, after a 12-year standoff, is aimed at easing fears that Tehran will develop nuclear weapons under the guise of its civilian activities, an ambition it hotly denies.

But a last-ditch diplomatic blitz in Vienna this week to secure a deal appeared to be unable to bridge major differences, forcing negotiators to question whether more time is a better option.

"Our focus remains on taking steps forward toward an agreement, but it is only natural that just over 24 hours from the deadline we are discussing a range of options both internally and with our P5+1 partners," a senior US State Department official said.

"An extension is one of those options. It should come as no surprise that we are also engaged in a discussion of the options with the Iranians," added the official.

"This does not mean that we are not continuing to discuss the broad range of difficult issues and working to make progress on all the issues that need to be part of a comprehensive agreement."

US Secretary of State John Kerry met Iranian counterpart Mohammad Javad Zarif on Sunday for the sixth time since Thursday in an attempt to break the deadlock. Neither commented publicly afterwards.

Russian Foreign Minister Sergei Lavrov, a key player who arrived Sunday afternoon, also met both Zarif and Kerry separately as well as German Foreign Minister Frank-Walter Steinmeier.

Britain and France's ministers had also arrived in Vienna while their Chinese counterpart was due early Monday.

"What a deal would do is take a big piece of business off the table and perhaps begin a long process in which the relationship not just between Iran and us but the relationship between Iran and the world, and the region, begins to change," US President Barack Obama in an ABC News interview aired Sunday.

- Gaps -

Diplomats on both sides say that the two sides remain far apart on the two crucial points of contention: uranium enrichment and sanctions relief.

Enriching uranium renders it suitable for peaceful purposes like nuclear power but also at high purities for the fissile core of a nuclear weapon.

Tehran wants to massively ramp up the number of enrichment centrifuges -- in order, it says, to make fuel for future reactors -- while the West wants them dramatically reduced.

Iran wants painful UN and Western sanctions that have strangled its vital oil exports lifted, but the powers want to stagger any relief over a long period of time to ensure Iranian compliance with any deal.

- Extension -

In view of the difficulties -- and of the dangers posed by a complete collapse -- many experts have long believed that the negotiators would put more time on the clock.

An Iranian source told AFP earlier Sunday, while stressing at that point that adding time was not yet on the table, that the extension "could be for a period of six months or a year."

Another extension -- as happened with an earlier deadline of July 20 -- carries risks of its own including possible fresh US sanctions that could lead Iran to walk away.

Arms Control Association analyst Kelsey Davenport told AFP that an extension of six months to a year "would not fly" with the other parties.

Any extension "will have to be very short because there are too many hardliners, particularly in Washington and Tehran, that want to sabotage this deal," she told AFP.

With nuclear deal in balance, hardliners protest in Tehran
Tehran (AFP) Nov 23, 2014 - The tensions over a possible nuclear deal involving Iran were laid bare Sunday outside an atomic facility in Tehran where a rare protest drew hardliners critical of the government's negotiators.

While the crowd was small -- about 200, mostly students, gathered at the entrance to the Tehran Research Reactor -- the event was the first such officially approved demonstration in months.

It coincided with the penultimate day of talks in Vienna between Iran and the United States and other world powers aimed at a permanent deal that seeks to block all routes to Tehran developing a bomb.

As a sign of defiance on Iran's nuclear activities, several protesters wore white lab coats in memory of four Iranian atomic scientists that Tehran says were assassinated by Israel and the United States.

"We protest the process of negotiations and the suspension of sanctions. Sanctions should be lifted altogether," said Hamed Tamanaie, one of the protesters.

"Nuclear energy is our absolute right," and "Sanctions won't stop us," read placards held by the protesters.

The crowd chanted "Death to America" while a speaker rounded on the conduct of the year-long negotiations which entered their final 36 hours with a deal hanging in the balance.

President Hassan Rouhani and Mohammad Javad Zarif, the foreign minister who is leading the Iranian side in the Austrian capital, "do not know how to do diplomacy", the speaker said.

One woman held a banner that said: "The centrifuges are not working, nor is the economy," alluding to Rouhani's pledge to restart talks to help Iran's sanctions-hit economy recover.

The protest highlighted the feelings of some Iranians that the government has already made too many concessions on its nuclear programme during the discussions of the past year.

- 'Impossible' deal -

However, an Iranian source in Vienna signalled openness to extending the talks by six months or even another year.

Such an extension would be under terms of an interim deal reached in Geneva last November that traded a temporary freeze on some of Iran's nuclear activities for limited sanctions relief, the source said.

The so-called P5+1 -- permanent UN Security Council members Britain, China, France, Russia and the United States, plus Germany -- have been locked in talks with Iran since February to turn the interim Geneva accord into a lasting agreement.

Such a deal is aimed at easing fears that Tehran could develop nuclear weapons under the guise of its civilian activities.

The Islamic republic denies it wants to build an atomic bomb and insists its nuclear programme is entirely peaceful.

Sunday's newspaper editorials in Tehran showed stark divisions on the merit of the talks.

"Our country's nuclear challenge with the P5+1 will never achieve results," said Kayhan, a hardline conservative daily whose editor-in-chief is directly appointed by Iran's supreme leader Ayatollah Ali Khamenei.

"Reaching an agreement which would bring to an end this 12-year challenge is not only far-fetched, but impossible," it added.

However, Shargh, a reformist paper that has supported Rouhani and Zarif, said there was "no going back to how it was", a reference to the tense situation on the nuclear issue under Iran's last government led by president Mahmoud Ahmadinejad.

"Even if no agreement is signed on Monday, this wouldn't mean that there has been no result or that the negotiations have reached an impasse," it said.