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Weary negotiators hoped Thursday to see the light at the end of the tunnel after talking until dawn, but midway through an eighth day were still haggling over the outlines of a deal curtailing Iran's nuclear programme.

US Secretary of State John Kerry and Iranian counterpart Mohammad Javad Zarif talked through the night, going line by line over their differences in a bid to agree a framework for an accord to cut back Iran's nuclear ambitions, diplomats close to the talks said.

They made "significant progress," but there is no "final result yet," Zarif told reporters early Thursday at the Swiss hotel hosting the negotiations, saying he felt "lucky" to have slept for two hours.

As German Foreign Minister Frank-Walter Steinmeier cancelled a planned trip to the Baltics to stay in Lausanne, Iran's nuclear chief Ali Akbar Salehi spoke of "lots of progress" and said that the "sunset is close".

But as negotiators from the powers, without Iran, after a break held a two-and-a-half hour meeting to review progress on Thursday morning, signs emerged that they were not in fact on the verge of a breakthrough.

"The conclusion is far from being imminent," one Western diplomat said.

After 18 months of intense negotiations, the six world powers - Britain, China, France, Germany, Russia and the United States - and Iran are hoping to agree a deal that puts a lid on 12 years of dangerously rising tensions.

The aim is to turn the framework they want to leave Lausanne with into a comprehensive accord backed by specific technical commitments by June 30 when an interim deal struck in November 2013 expires.

'Three main issues' 

The global powers originally set a midnight Tuesday deadline, later extended, to agree the outlines of a deal that they will then try to finalise by 30 June. Only then would Iran receive sanctions relief, diplomats said. 

However, even before a deal is sewn up, opponents have been lining up to criticise it.

These include US President Barack Obama's Republican opponents and Israel's Benjamin Netanyahu, who launched a blistering attack on the "dangerous" mooted deal.

Israel is the sole, if undeclared, nuclear-armed power in the Middle East and has long been opposed to any Iran accord.

Saudi Arabia, leading an Arab coalition of airstrikes on Shia rebels in Yemen is also uneasy about any US-Iran thawing of ties.

'A jigsaw puzzle' 

Western diplomats say some areas in a highly complex jigsaw puzzle of an accord are tentatively agreed. But they caution there is a long way to go. 

One said that Iran had "more or less" agreed to slash the number of its centrifuge enrichment machines from 20,000 to 6,000 and to ship abroad most of its stockpile of low-enriched uranium.

This would make it a much more lengthy process to further purify these stocks to weapons-grade. Iranian officials dismissed the numbers as "speculation", with Araghchi ruling out sending the stocks abroad, although he said "other options" were being examined.

This could include diluting low-enriched uranium or converting it to another form. 

But Iranian officials have expressed guarded optimism that a breakthrough may be at hand.