Iran and world powers missed a Monday deadline to clinch a landmark nuclear deal and defuse a 12-year standoff but gave themselves seven more months to reach agreement.

The failure followed an intensive five-day diplomatic push in the Austrian capital Vienna involving the foreign ministers of Iran, the United States, Russia, China, Britain, France and Germany.

But US Secretary of State John Kerry, speaking in Vienna, and Iranian President Hassan Rouhani, speaking in Tehran, said real progress had been made in the talks and raised hopes a deal could eventually be sealed.

"This path of negotiation will reach a final agreement," Rouhani said on state television. "Most of the gaps have been removed."

In their second extension this year, Iran and the five permanent members of the UN Security Council plus Germany, known as the P5+1, will seek to strike an outline deal by March and to nail down a full technical accord by July 1, officials said.

"These talks aren't going to suddenly get easier just because we extend them," Kerry said as he and other officials conceded a midnight Monday deadline would be missed.

"They are tough. They have been tough and they are going to stay tough," he told hundreds of journalists crowded into a tent outside the 19th century palace where the talks were held.

"But in these last days in Vienna we have made real and substantial progress and we have seen new ideas surface. And that is why we are jointly, the P5+1 six nations and Iran, extending these talks for seven months."

In the best chance to resolve the standoff over Iran's nuclear programme, the P5+1 world powers have been for months seeking to turn an interim deal into a lasting accord.

Such an agreement is aimed at easing fears that Tehran will develop nuclear weapons under the guise of its civilian activities, an ambition Iran denies.

It could see painful sanctions on Iran lifted, silence talk of war and usher in a new era of cooperation between Washington and Tehran, which have called each other the "axis of evil" and the "Great Satan."

A deal could begin a process in which the "relationship between Iran and the world, and the region, begins to change," US President Barack Obama said in an ABC News television interview Sunday.

But a last-ditch diplomatic blitz in Vienna in recent days involving Kerry and the other foreign ministers failed to bridge the remaining gaps.

This included eight meetings since Tuesday between Kerry and his Iranian counterpart, Mohammad Javad Zarif, and numerous other gatherings in the Austrian capital.

"Despite good conditions, despite a very constructive negotiating atmosphere, we didn't get as far as we would have wished," said German Foreign Minister Frank-Walter Steinmeier.

- Gaps on crucial points -

Diplomats say that, despite some progress, both sides remain far apart on two crucial points: uranium enrichment and sanctions relief.

Enriching uranium renders it suitable for peaceful purposes like nuclear power. But at high purities it is also used as 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 a fleet of power reactors that it is yet to build.

The West wants the enrichment dramatically reduced, a move which together with more stringent UN inspections and an export of Iran's uranium stocks, would make any attempt to make the bomb all but impossible.

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 to ensure Tehran complies with any deal.

- More time on the clock -

Many experts long believed that the negotiators would have to put more time on the clock.

The conditions set by last November's interim deal will remain in place until July, including a continued freeze by Iran of contentious parts of its nuclear activities.

In return, Iran will keep receiving around $700 million (560 million euros) in frozen funds per month, Hammond said, or $4.9 billion by July, adding to some $7 billion received since January.

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

"New sanctions legislation against Iran, which has been proposed by more than several members of Congress, would undermine the chance to reach a comprehensive deal that guards against a nuclear-armed Iran," Arms Control Association analyst Kelsey Davenport told AFP.

"The imposition of new sanctions measures will most certainly provoke Iran to take escalatory measures that could lead to a larger crisis in the Middle East."

Kerry appealed on Monday on US lawmakers -- with Republicans in control of both houses from January -- not to pass fresh sanctions on Iran.

"This is certainly not the time to get up and walk away... We look for your support (in Congress) for this extension," Kerry said.

"We would be fools to walk away from a situation where the breakout time (to make a nuclear weapon) has already been expanded... and where the world is safer."

Iran nuclear talks: more than a decade of crisis
Vienna (AFP) Nov 24, 2014 - After talks between Iran and world powers on its nuclear programme missed yet another deadline Monday, here is a summary of the main developments in the crisis since 2002.

2003

- August: The International Atomic Energy Agency (IAEA) reveals that traces of enriched uranium have been identified in Natanz, central Iran, after an Iranian exiled opposition group in 2002 revealed the existence of a nuclear installation.

- October: Following an unprecedented visit by foreign ministers from Britain, France and Germany, Iran suspends uranium enrichment activities but later vows it will never renounce its nuclear programme.

2005

- August 8: After the election of hardline president Mahmoud Ahmadinejad, Tehran resumes uranium enrichment in Isfahan, central Iran.

2006

- January: The UN Security Council's five permanent members -- Britain, China, France, Russia and the United States -- agree to have the IAEA present the issue to the full Council.

- June 6: The permanent members plus Germany, dubbed P5+1, propose a framework for talks but Iran later rejects the offer.

- December 23: The UN imposes the first of several rounds of sanctions on Iran's trade in sensitive nuclear materials and technology.

2007

- November 7: Iran says it has at least 3,000 centrifuges for enrichment, which in theory would allow it to produce enough enriched uranium for a nuclear bomb in less than a year.

2009

April 8: Following the November election of US President Barack Obama, world powers offer to resume negotiations with Tehran, which later declares major advances in its nuclear drive as Ahmadinejad opens a uranium enrichment site in Isfahan.

- September 25: Western nations reveal the existence of a previously undeclared enrichment site inside a mountain in Fordo.

- October 1: Negotiations resume, with an agreement in principle for Iranian uranium to be enriched abroad, but a final deal is not reached.

2010

- February 9: Iran says it has begun to enrich uranium to 20 percent at Natanz, close to the level required for a nuclear weapon.

2011

- November 8: The IAEA points to a possible military dimension to Iran's nuclear activities.

2012

- January 23: After talks break down again, the EU slaps an embargo on Iranian oil exports.

2013

- August 6: Newly-elected President Hassan Rouhani says Tehran is ready for "serious" negotiations.

- September 27: Rouhani reveals he and Obama have spoken by telephone in the highest-level contact between the two countries since 1979.

- October 14: Negotiations resume

- November 24: Iran agrees to curb its nuclear programme in exchange for sanctions relief in a preliminary deal which comes into force on January 20.

2014

- February 18: Negotiations start on a permanent accord.

- July 18: Iran and world powers agree to extend the deadline for a permanent deal to November 24.

- November 5: Obama says the US has put forward a "framework" plan to meet Iran's energy needs. Two days later the IAEA says Iran is still failing to provide answers in its probe into possible military dimensions.

- November 11: Russia signs a deal with Iran to build two new nuclear reactors and agrees to expand the total to nine.

- November 18: Iran and the six world powers begin a final round of talks.

- November 24: Iran and world powers fail in a push to seal a landmark nuclear deal, giving themselves until March 1 to agree an outline accord and until July 1 -- or seven more months -- to reach a full technical accord.