Iran's Supreme Leader, Ali Khamenei, has raised doubts about the success of ongoing nuclear negotiations with the United States, warning against "excessive and outrageous" American demands regarding uranium enrichment, he said on May 20.

In an event marking the death anniversary of former President Ebrahim Raisi and Foreign Minister Hossein Amir Abdollahian, who died in a helicopter crash close to the border of Azerbaijan, Khamenei appeared downbeat on upcoming negotiations, indirectly criticising the current administration’s efforts to engage with Trump following his recent string of deals with regional rivals. 

"I don't think nuclear talks with the US will bring results. I don't know what will happen," Khamenei said during the ceremony.

The Supreme Leader warned the American side in the indirect negotiations to "refrain from talking nonsense," specifically criticising US demands on enrichment.

"Saying they won't allow Iran to enrich uranium is excessive and outrageous. No one is waiting for permission to do this or that. The Islamic Republic has its policy, its method, and pursues its policies," Khamenei said.

He promised to explain to the Iranian nation on another occasion "why they insist so much on enrichment, why Western and American parties and others insist that there should be no enrichment in Iran."

Speaking about the late President Raisi, Khamenei recalled how in his first interview as president, when asked if he would negotiate with America, "he clearly said 'no' without any ambiguity, and he didn't in direct reference to current President Masoud Pezeshkian’s outreach to the US and Western countries.

 He didn't allow the enemy to say that through threats, enticements, or trickery, they were able to bring Iran to the negotiating table."

The Supreme Leader noted that while indirect negotiations took place during Raisi's tenure, as they do now, they were "without results." He added, "Now too, we don't think they will lead to results. We don't know what will happen."

Despite expectations that a fifth round of negotiations might occur over the weekend in Rome, talks appear to be on shaky ground as both sides clash over the uranium enrichment issue.

Deputy Foreign Minister Majid Takht-Ravanchi said on May 19 that talks would fail if Washington insists Tehran refrain from domestic enrichment, which the US claims could lead to developing nuclear weapons. Iran maintains that its nuclear programme is entirely peaceful.

Another Deputy Foreign Minister, Kazem Gharibabadi, said Tehran had received and was reviewing a proposal from the United States. Last week US President Donald Trump warned Iran it needed to "move quickly or something bad is going to happen."

US President Donald Trump, who withdrew from the 2015 nuclear deal during his first term and reimposed sweeping sanctions, has repeatedly warned Iran it would face bombing and severe sanctions if it failed to reach a compromise on its nuclear programme.