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."
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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.