Iran's supreme leader Ayatollah Ali Khamenei said on Thursday that last week's hard-won framework accord with world powers was no guarantee of a final nuclear deal and that much work remained.
And President Hassan Rouhani said the Islamic republic would not sign any final agreement unless "all economic sanctions are totally lifted on the same day".
"What has been done so far does not guarantee an agreement, nor its contents, nor even that the negotiations will continue to the end," said Khamenei, who has the final word on all matters of state, according to his official website.
After a week of gruelling last-ditch negotiations, Tehran and the six powers agreed on April 2 on the framework of a deal, to be finalised by the end of June, reining in Iran's nuclear programme in return for the lifting of international sanctions.
"Everything is in the detail, it may be that the other side, which is unfair, wants to limit our country in the details," Khamenei said, in his first comments on the deal.
Playing down expectations of a deal after the interim accord -- which sparked celebrations in the streets of Iranian cities -- Khamenei said he had not taken any position until now as "there is nothing to take a stance on".
"Officials say that nothing has been done yet and there is nothing binding. I am neither for nor against."
Under the outline text agreed in the Swiss city of Lausanne between Tehran and the so-called P5+1 powers -- the United States, Britain, China, France and Russia plus Germany -- Iran must significantly reduce its number of centrifuges in exchange for a suspension of sanctions.
The outline was a major breakthrough in a 12-year international crisis over Iran's nuclear programme.
"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."
He said that retaining a civil nuclear industry in any agreement was vital for Iran's future development.
"The nuclear industry is a necessity, for energy production, for desalination, and in the fields of medicine, agriculture and other sectors," he said.
'Sanctions should be lifted same day'
In a potential obstacle to any final deal, President Rouhani said his country wanted sanctions lifted on the day of the implementation of any agreement.
"We will not sign any agreements unless on the first day of the implementation of the deal all economic sanctions are totally lifted on the same day," he said.
The pace at which the sanctions will be lifted is one of the outstanding issues that still has to be agreed in the final accord.
Western governments, which have imposed their own sanctions over and above those adopted by the United Nations, have been pushing for it to happen only gradually.
"In return for Iran's future cooperation, we and our international partners will provide relief in phases from the sanctions that have impacted Iran's economy," US Secretary of State John Kerry said last week.
Rouhani, who was speaking on Iran's National Nuclear Technology Day, reiterated that his government remained determined to develop its civil nuclear programme.