Negotiators failed for a second time this year to resolve the 12-year dispute over Iran's nuclear ambitions and have extended talks for another seven months.

The five permanent members of the UN Security Council and Germany have been locked in talks with Iran for months over concerns the country is developing nuclear weapons.

They were seeking to turn an interim deal into a lasting accord, but the deadline expired today.

After a frenetic six days of diplomacy in Vienna, negotiators are now hoping to secure a general agreement by March and a final deal by June 30.

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International negotiators are concerned that Iran is using its nuclear development programme as a cover for developing nuclear weapons and they have imposed economic sanctions on Tehran.

Iran denies the charge, saying it is only interested peaceful nuclear programs like producing power.

The country's nuclear programme has been left frozen but intact, without any of the cuts sought by the US.

British Foreign Secretary Philip Hammond said the sides were giving themselves until March to agree on a text 'that sets out in layman's language what we have agreed to do.'

The five permanent members of the UN Security Council and Germany have been locked in talks with Iran for months (From left, French Foreign Minister Laurent Fabius, U.S. Secretary of State John Kerry, former EU Foreign Policy Chief Catherine Ashton, British Foreign Secretary Philip Hammond and German Foreign Minister Frank-Walter Steinmeier)

Experts then will be given another four months to 'translate that into precise definitions of what will happen on the ground,' he told reporters.

U.S. Secretary of State John Kerry focused on praise, in an apparent attempt to maintain a relatively cordial atmosphere at the negotiating table.

Mr Kerry, who arrived Thursday and met repeatedly with Iranian Foreign Minister Mohamad Javad Zarif, said Zarif 'worked diligently and approached these negotiations in good faith.'

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'We have made real and substantial progress and we have seen new ideas surface,' he said.

'Today we are closer to a deal that will make the whole world, especially our allies in Israel and the Gulf, safer. We would be fools to walk away.'

Mr Hammond spoke of 'significant progress,' while German Foreign Minister Frank-Walter Steinmeier said only differences about 'technical details' remained.

But the length of the extension suggested that both sides felt time was needed to overcome the disputes on how much Iran needed to restrict nuclear activities.

'All the people involved here feel that there really is a chance to find out a way to each other and we are going to take that chance,' Mr Steinmeier said.

Members of the new Republican-controlled U.S. Congress that will be sworn in in January have already threatened to impose additional sanctions on Iran.

It may well have enough votes to overturn an expected veto of such legislation by President Barack Obama.

New sanctions could very well derail the talks, as Iran has signaled they would be a deal breaker, and Kerry appealed to Congress to 'support ... this extension.'

In Tehran, hardliners fearful that their country could give away more than it gets under any final deal could increase pressure on Supreme Leader Ayatollah Ali Khamenei to break off talks.

The talks extension, however, appears to have the approval of Khamenei, who is the ultimate arbiter in his country.

Protests: An Iranian student holds a placard to show a support for Iran's nuclear program in a gathering in front of the headquarters of Iran's Atomic Energy Organization in Tehran, Iran, yesterday

Both sides are haggling over how many - and what kind - of centrifuges Iran should be allowed to have.

The machines can enrich uranium from low, reactor-fuel level, up to grades used to build the core of a nuclear weapon, and their output grows according to how modern they are.

Washington wants deeper and more lasting cuts in the program than Tehran is willing to give.

Prime Minister of Israel Benjamin Netanyahu has threatened the use of force against Iran several times since 2009 and three years ago even asked his cabinet to authorise such a strike.

Iran's nuclear effort has expanded significantly since.