Iran's chief negotiator warned the United States is not to be trusted yesterday, saying Tehran would not agree to any deal with Washington unless it fully secures Iranian rights.
Mohammad Bagher Ghalibaf's remarks came as reports emerged that US President Donald Trump had sent a tougher peace proposal back to Iran, and underlined the rift that the parties still need to close.
Any further tweaks to the draft could further delay an agreement to formally end the Middle East war and reopen the Strait of Hormuz after weeks of fraught negotiations marked by sharp rhetoric and occasional flare-ups of violence.
The New York Times and Axios reported that Trump had sent back a "tougher" new framework to be considered by Iran, though details remain unclear.
Trump has said his priorities include stopping Iran from any nuclear weapon development and re-opening the Hormuz shipping lane, over which Iran has sought to impose control since the war began.
"The one guarantee that I have to have is that there will be no nuclear weapons. They've agreed to that, and it was very interesting," he told his daughter-in-law Lara Trump in an interview on her Fox News show.
But Tehran has previously cast doubt on Trump's assertions and the sides remain far apart on key issues.
"We will not approve any agreement until we are certain that the rights of the Iranian people have been upheld," Ghalibaf said, in a video broadcast on state television.
According to the Tasnim news agency, "exchanges between Iran and the United States regarding the text of a possible memorandum of understanding are ongoing, with both parties regularly proposing amendments.
"No agreement has yet been finalised, and it is possible that any agreement will be rejected," it said.
Iran has said it needs the release of $12bn in frozen assets before engaging in substantive talks on its nuclear program, dismissing earlier Trump comments that its enriched uranium stockpile would be destroyed as "baseless", according to Iranian media.
Tehran has also insisted that Lebanon be included in any deal, despite ongoing fighting, with Beirut accusing Israel of a "scorched-earth policy" as it expands operations against Hezbollah.
Though daily strikes throughout Iran and the Gulf halted after Tehran and Washington struck a temporary ceasefire in April and talks mediated by Pakistan, sporadic fighting has continued.
Iran's Revolutionary Guards had shot down a US military drone "about to enter Iranian territorial waters", Iran's state broadcaster IRIB reported, though Washington has not confirmed the incident.