Iran’s Supreme Leader Ayatollah Ali Khamenei says that there is “no guarantee” of a final deal with world powers on its nuclear programme.
Earlier, President Hassan Rouhani said Iran would not sign a final deal on its nuclear programme unless sanctions were lifted “on the first day”.
Last week Iran and world powers reached a framework agreement on the issue.
Since then, Iranian and US officials have been trying to persuade hardliners in both countries to back the deal.
The deal stipulates Iran must slash its stockpile of enriched uranium that could be used in a nuclear weapon, and cut by more than two-thirds the number of centrifuges that could be used to make more.
In return, UN sanctions and separate measures imposed unilaterally by the US and EU will be gradually suspended as the global nuclear watchdog, the International Atomic Energy Agency (IAEA), confirms Iranian compliance.
In a statement on his website, Mr Khamenei said “it is possible that the untrustworthy side [the six world powers who negotiated the agreement] wants to restrict our country in the details”.
“I have never been optimistic about negotiating with America. While I was not optimistic, I agreed with this particular negotiation and supported the negotiators,” the statement says.