Iran's Supreme Leader Ayatollah Ali Khamenei has stopped short of giving his endorsement to the framework nuclear deal struck last week between Tehran and world powers.
It comes as the country's president warned separately that Tehran's approval of a final deal depends on the immediate lifting of all sanctions related to its controversial nuclear programme.
The comments, taken together, could represent simply a tough bargaining stance by the Islamic Republic ahead of the next round of negotiations in the countdown to the final deal expected by June 30.
But President Hassan Rouhani's demand of an immediate and full sanctions relief is likely to complicate efforts to reach a final deal.
"We will not sign any agreement unless all economic sanctions are totally lifted on the first day of the implementation of the deal," Mr Rouhani said during a ceremony marking Iran's nuclear technology day, which celebrates the country's nuclear achievements.
Iran and the six world powers agreed last week in Switzerland on a framework deal, which is meant to curb Iran's bomb-capable technology while giving Tehran quick access to bank accounts, oil markets and financial assets blocked by international sanctions.
But the framework deal does not include the immediate lifting of punitive sanctions imposed on Iran. Instead, it says the sanctions will be suspended once international monitors verify that Tehran is abiding by the limitations spelled out in the agreement.
The deal also specifies that if at any time Iran fails to fulfil its commitments, these sanctions would snap back into place.
The framework agreement has received endorsement by much of the Iranian establishment, though hard-liners have overwhelmingly opposed it and described the deal as a "defeat" for Iran.
Mr Khamenei said in remarks published on his official website that he "is neither for nor against" the deal.
Mr Khamenei, who has the final say on all state matters, added that there "is no binding" agreement yet. He also said that the punitive "sanctions should be lifted completely, on the very day of deal" - something that has not been agreed on in Switzerland.
The top leader also cautioned that the six world powers - five permanent UN Security Council members and Germany - are "not to be trusted" and may try "to limit Iran" in further talks.
A lot remains to be done until the deal is finalised, Mr Khamenei said, adding it may take more than three months.
"The problems are in the details and (negotiators) should sit down and discuss them one by one," he said.
If the deal is successfully finalised, Mr Khamenei added it would show that negotiations are possible on other issues beyond the nuclear programme. But the talks in Switzerland "are only about nuclear case", he said. "We do not have talks with the United States on any other issue, for the time being."
Mr Khamenei also urged Iranian negotiators not to accept any "unconventional inspections" of Iran's nuclear facilities - presumably meaning sudden or unannounced inspections - and stressed that inspections of military facilities would not be permitted.
"No way, we should not allow them to infiltrate security and defensive installations," said Mr Khamenei.
The West has long feared Iran's nuclear programme could allow it to build an atomic bomb and that Tehran has used uranium enrichment - the key aspect point in the negotiations and a possible pathway to nuclear arms - to pursue nuclear weapons. Iran denies the charge, saying its nuclear programme is for peaceful purposes only, such as power generation and cancer treatment.
Mr Rouhani described the framework deal reached in Switzerland as evidence that Iran has "not surrendered to a policy of pressure, sanctions and bullying".
"This is our victory," said Mr Rouhani.