Tehran: Iran's leaders plunged the hard-won framework accord on their nuclear program into doubt on Thursday, warning they may not sign a final deal and would demand immediate sanctions relief.

Supreme Leader Ayatollah Ali Khamenei's response to last week's outline deal had been keenly awaited, and came as a blow to supporters of the plan to rein in Tehran's nuclear ambitions.

"What has been done so far does not guarantee an agreement, nor its contents, nor even that the negotiations will continue," said Khamenei, who has the final word on all matters of state.

Separately, but in another setback for the deal, President Hassan Rouhani said Iran would not sign a final agreement unless "all economic sanctions are totally lifted on the same day."

This drew a rebuke from the United States, one of the six world powers negotiating with Iran on the accord, which warned sanctions relief would be a gradual process once the deal was done.

US State Department spokesman Jeff Rathke said: "Sanctions will be suspended in a phased manner upon verification that Iran has met specific commitments under a finalised joint comprehensive plan of action."

Britain's Foreign Office backed this position.

"Sanctions will remain in place until the comprehensive deal is agreed and there is IAEA-verified implementation by Iran of its nuclear commitments," a spokesman said.

Rapturous response

On April 2, after months of gruelling negotiations, Tehran and the six powers agreed on the broad outline of a deal to impose tighter controls on Iran's nuclear programme in exchange for lifting economic sanctions.

Thousands of Iranians flooded into the streets to welcome the agreement, which they hope will end decades of political and economic isolation, but the Islamic republic's conservative leadership has been more reserved.

The P5+1 powers and Tehran have given themselves until June to finalise a detailed accord, but Washington has released fact sheets outlining steps it says Tehran has already agreed to take.

This has angered Iran, and drew a fierce response from Khamenei.

In his first comments on the outline, Khamenei said "everything is in the detail; it may be that the other side, which is unfair, wants to limit our country in the details."

Seeking to dampen domestic public expectations after the rapturous response to the apparent breakthrough, Khamenei said "there is nothing binding. I am neither for nor against."

Under the outline deal, Iran must slash the number of its nuclear centrifuges in exchange for a suspension of economic sanctions.

Centrifuges enrich uranium to a level at which it can fuel power plants or, at greater levels of purity, form the core of a nuclear bomb.

The outline was seen as a major breakthrough in a 12-year international crisis over Iran's nuclear programme, which Western powers allege is a quest to build an atomic weapon.

Sanctions lifted 'same day'

"I have always supported and still support the Iranian negotiating team," Khamenei said.

"I welcome any agreement that protects the interests and greatness of the nation, but having no agreement is more honourable than an agreement in which the interests and greatness of the nation is damaged."

And he insisted that retaining a civil nuclear industry is vital for Iran's future development.

Rouhani's intervention also appeared likely to slow progress towards a final accord, as the pace at which the sanctions will be lifted is one of the issues that still has to be agreed.

Western governments, which have imposed their own sanctions over and above those adopted by the United Nations, want Iran to return to the international fold only gradually.

Rouhani, speaking on Iran's National Nuclear Technology Day, also said his government remains determined to develop a civil nuclear programme.

The nuclear stand-off is only one of the issues clouding US President Barack Obama's attempt to thaw the decades-old conflict with Iran.

Tehran stands accused of destabilising its Arab neighbours through sponsoring armed groups like Lebanon's Hezbollah, Shiite Islamist fighters in Iraq and the Houthi movement in Yemen.

Washington's top allies in the region, Israel and Saudi Arabia, are deeply sceptical of any deal that might see warmer contacts between Iran and the United States.

'Distortion by US'

Although he claimed to be "100 per cent" supportive of the nuclear accord if it resulted in immediate removal of all sanctions and did not authorise any supervision over military sites, the ayatollah failed to give his full support to the framework that Tehran agreed with six big world powers - the US, UK, France, Russia, China and Germany - to limit its nuclear activities in return for removal of many sanctions.

"I neither oppose nor agree with it [the agreement]," he said during a speech on Thursday. He added that "congratulations are meaningless" when "there is nothing binding yet" and "there is still no guarantee of reaching the finishing line".

The ayatollah's comments may be a tactical move to maintain Iran's bargaining power over details to be worked out by the end of June -- the deadline to reach a final comprehensive accord, but they are a sign of how difficult it will be to achieve a final agreement in under three months.

They will also make it harder for the Obama administration to convince the US Congress that the talks are making considerable progress.

"All problems are from now on," he said. "The other side (the US) -- which is used to violating commitments, is stubborn and has a record of stabbing (Iran) in the back - may try to surround the Iranian nation and negotiators over details."

The US has been insistent that sanctions will be lifted only once Iran has fulfilled its commitments under any final deal.

The US fact sheet released shortly after the announcement also specified that sanctions relief was partly dependent on Iran addressing concerns about military aspects of its nuclear programme, which would be likely to require visits to certain sensitive sites.

With Congress scrutinising the talks closely, the fact sheet is a central element in the administration's push to win political support for the final stage of the negotiations.

If the announcement last week had only included the broad statement by the parties, pressure in Congress for new legislation to block a deal with Iran would probably be even stronger.

However, the supreme leader insisted that "sanctions should be completely lifted on the first day of the agreement, otherwise why would we have been negotiating?

No to inspections

"Also, under no conditions . . . authorities should give permission for aliens to have access to our defence and security spheres."

Iran's senior politicians and military figures have supported the nuclear agreement. But the negotiating team, led by Mohammad Javad Zarif, foreign minister, is coming under mounting domestic pressure to disclose details, in particular on when and how sanctions will be lifted.

Rouhani reiterated on Thursday his team would not sign any agreement which would not lead to lifting of sanctions.

He has repeatedly said sanctions will stop "on the first day of implementation of the agreement", but has not differentiated ceasing to implement sanctions and lifting them, which may take longer than many Iranians hope.

Some Iranian MPs have called on Zarif to release an Iranian fact sheet of the framework that lists Iran's successes in the negotiations to counter the US version, which lists the regime's concessions.

Khamenei has urged Iran's negotiating team to make more details of the talks public and added that if an accord were achieved and the US remained committed, Iran could "use the experience" and negotiate with Washington over other differences -- a rare indication that Iran could co-operate with its arch-enemy -- presumably in the region.

But he cautioned that "for now" the focus was only on the nuclear programme.