The US abandoned late Tuesday a midnight deadline to agree the outlines of a nuclear deal with Iran but insisted that "enough progress" merited extending marathon talks into Wednesday.

"We've made enough progress in the last days to merit staying until Wednesday. There are several difficult issues still remaining," State Department spokeswoman Marie Harf said.

The announcement came late on a sixth day of talks in Switzerland aimed at laying the groundwork for a deal that world powers hope will prevent Iran developing nuclear weapons under the guise of its civilian programme.

The stakes are high, with fears that failure to reach a deal may set the United States and Israel on a road to military action to thwart Iran's nuclear drive, which Tehran says is purely peaceful.

Earlier Chinese Foreign Minister Wang Yi left the crunch talks in Lausanne as his French counterpart Laurent Fabius said that the negotiations were "complicated... long and difficult".

A German diplomatic source also said the talks were "difficult" with a "changeable atmosphere" and "frequent breaks to negotiate in smaller groups".

A Western diplomat said the army of technical and sanctions experts would continue plugging away "for (the) next hours. All parties (are) working hard and (are) committed to finding a solution."

Senior Iranian negotiator Hamid Baidinejad said: "The negotiations will end when solutions have been found... We are ready to continue. We are not watching the clock."

The return earlier to Lausanne of Russian Foreign Minister Sergei Lavrov had brought a ray of optimism, having said in Moscow before leaving that the chances of an accord were "high".

- Low on detail? -

Under a deal to be finalised by June 30, the powers want Iran to scale back its nuclear programme to give the world ample notice of any dash to make the bomb by extending the so-called "breakout" time.

In return, the Islamic republic is demanding the lifting of sanctions that have strangled its economy.

But the question is how much detail will be in the framework accord that Iran and the six powers -- the US, China, Russia, Britain, France and Germany -- want to leave Lausanne having secured.

If it falls short of firm commitments by Iran then US President Barack Obama will find it hard to fend off attempts by his Republican opponents to pass fresh sanctions on Tehran.

Iran's negotiators are also under pressure from their own domestic hardliners not to give too much away and for President Hassan Rouhani to deliver on his promises to secure the lifting of sanctions.

Fresh US sanctions could therefore torpedo the whole negotiating process that was launched after Rouhani became president in 2013.

Republicans fear that since some of its nuclear infrastructure will likely stay intact, Iran will still be able to get the bomb -- a concern shared by Israeli Prime Minister Benjamin Netanyahu, whose country is widely assumed to have nuclear weapons itself.

"The greatest threat to our security and our future was and remains Iran's attempt to be armed with nuclear weapons. The agreement being formulated in Lausanne paves the way to that goal," Netanyahu told parliament Tuesday.

"We will do everything to protect our security and our future," he said in his third attack on the nuclear talks with Iran in as many days.

Saudi Arabia, which has led an Arab coalition bombing Iran-backed rebels in Yemen in recent days, is also alarmed by what is unfolding in Lausanne.

Its Foreign Minister Prince Saud al-Faisal called on the six powers not to "short-circuit the interests of the states of the region by dangling before Iran benefits that it will not be able to reap without cooperation with the countries of the region."

- Some areas sewn up -

Some areas of the mooted deal, including the future size of Iran's uranium enrichment capacity -- a process for making nuclear fuel but also the core of an atomic bomb -- appear to have been tentatively sewn up.

But the two sides still appear wide apart on other areas, including what to do with Iran's stockpiles of nuclear material and the pace at which sanctions would be eased.

The six powers are only prepared to suspend sanctions, not terminate them, in order to be able to put them back into place if Tehran violates the deal.

Other tricky issues include the duration of any accord, with Iran resisting demands by the powers to submit to ultra-tight inspections by the UN atomic watchdog for at least a decade.

Iran's 12-year nuclear standoff with the world
Lausanne (AFP) March 31, 2015 - World powers were grappling Tuesday to agree by midnight the outlines of a potentially historic deal with Iran to cut back its nuclear ambitions.

Here is a summary of the main developments in the Iran nuclear crisis since 2002:

2002-2004: Undeclared nuclear sites

After nuclear sites are uncovered in Natanz and Arak in central Iran in August 2002, Tehran agrees to undergo an inspection by the International Atomic Energy Agency (IAEA). The UN watchdog reveals that traces of enriched uranium have been identified.

On October 21, 2003, 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.

2005-2008: Enrichment to 3.5 percent

On August 8, 2005, after the election of hardline president Mahmoud Ahmadinejad, Tehran resumes uranium enrichment. European nations break off negotiations.

In January 2006, 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.

They are defied by Iran, which on April 11 says that it has enriched uranium to 3.5 percent purity. It then rejects an offer from the permanent UN members plus Germany, dubbed P5+1, on a framework for talks.

On December 23, 2006 the UN imposes the first of several rounds of sanctions on Iran's trade in sensitive nuclear materials and technology. It is followed by the United States and European Union.

Negotiations come to a standstill. On November 7, 2007 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. Today it has nearly 20,000, of which half are active.

2009-2012: Enrichment to 20 percent

In 2009, 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 conversion facility on April 9 in Isfahan.

Then, on September 25, Western nations reveal the existence of a previously undeclared enrichment site inside a mountain in Fordo.

On February 9, 2010, after the breakdown of talks on enriching uranium in a third country, Iran says it has begun to enrich uranium to 20 percent at Natanz, close to the level required for a nuclear weapon.

With Israel threatening a preventative strike, the IAEA on November 8, 2011 points to a possible military dimension to Iran's nuclear activities.

On January 9, 2012, the IAEA says that Iran has started to enrich to 20 percent at Fordo.

On January 23, 2012, the EU agrees to slap an embargo on Iranian oil exports and freezes the assets of Iran's central bank.

Negotiations between Iran and the P5+1 resume in April after being stopped for 15 months.

2013: Preliminary accord

On August 6, newly-elected Iranian President Hassan Rouhani says Tehran is ready for "serious" negotiations.

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

On November 24, after intense negotiations Iran agrees to curb its nuclear programme in exchange for sanctions relief in an interim deal which comes into force on January 20.

2014: Extended negotiations

On February 18, negotiations start on a permanent accord, but despite intense diplomatic efforts fail and deadlines are extended twice.

The interim accord is extended. Iran says on August 27 it has started to modify the design its Arak heavy water reactor and limit plutonium output.

2015: Towards a permanent accord?

Negotiations restart in January with a deadline of a political accord by March 31. The framework deal, if it can be hammered out, is meant to be finalised with comprehensive technical annexes by June 30.

In the United States, Republicans take control of the Congress and threaten Iran with preventative sanctions, which Obama vows to veto.

On March 3, Israel steps up its campaign against a "dangerous" deal with Israeli Prime Minister Benjamin Netanyahu blasting the accord in a controversial speech to Congress.

On March 26, top US diplomat and Iranian Foreign Minister Mohammad Javad Zarif meet in Lausanne at the start of a flurry of talks for the outlines of a political accord.