Iran and world powers failed in an enormous diplomatic push to seal a landmark nuclear deal by a Monday deadline, deciding instead to give themselves seven more months to reach agreement.
In their second extension this year, Iran and the five permanent members of the UN Security Council plus Germany 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," US Secretary of State John Kerry said after five days of frenzied diplomacy in Vienna working towards a deal by midnight on Monday.
"They are tough. They have been tough and they are going to stay tough," he told a news conference.
"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 12-year 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 represent a much-needed success for both US President Barack Obama and his Iranian counterpart Hassan Rouhani.
A deal could begin a process in which the "relationship between Iran and the world, and the region, begins to change," Obama said in an ABC News television interview Sunday.
But a last-ditch diplomatic blitz in Vienna in recent days involving Kerry and other foreign ministers failed to bridge the remaining major differences.
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 -
In view of the difficulties, many experts long believed that the negotiators would 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."