Critical US-Iran nuclear talks on shaky ground

The future of high-stakes negotiations over Iran's nuclear program remains shrouded in uncertainty as the U.S. and Iran publicly clash over the terms of an interim agreement proposed by the Trump administration that is intended to pave the way for a longer-lasting deal.

Iran's supreme leader, Ayatollah Ali Khamenei, slammed that proposal during an address on Wednesday, saying it "contradicts our principle of power by 100%" and vowing that Iran would not agree to stop enriching uranium on its own soil, calling it "a key tool in the nuclear program."

The Iranian foreign minister, Seyed Abbas Araghchi, succinctly underscored Iran's terms in a post on X shortly after the Ayatollah's speech, writing "no enrichment, no deal."

Iran's stance contrasts sharply with that of President Donald Trump, which he emphasized on his social media platform on Monday.

"Under our potential Agreement -- WE WILL NOT ALLOW ANY ENRICHMENT OF URANIUM!" Trump wrote on Truth Social.

While the president and his Cabinet officials have repeatedly said Iran would be required to end enrichment activities under the terms of a deal, U.S. officials say that the initial interim proposal -- which is only intended to be a stepping stone to a broader agreement -- did not prohibit Iran from enriching uranium at low levels while a long-term solution to fuel Iran's civilian nuclear energy program could be established.

Trump's post appeared to contradict the position staked out by his negotiators, and although Iran has not yet formally responded to the interim proposal, it remained unclear on Wednesday whether the U.S. would stand by all the terms in its initial offer.

Whether delegations from Iran and the U.S. would meet for a sixth round of nuclear negotiations also remained unclear, although a U.S. official said a possible meeting in the coming days was under discussion.

After holding a phone call with Russian leader Vladimir Putin on Wednesday, Trump signaled that he would look to Moscow to play a greater role in negotiations with Iran.

"President Putin suggested that he will participate in the discussions with Iran and that he could, perhaps, be helpful in getting this brought to a rapid conclusion," Trump wrote on Truth Social. "It is my opinion that Iran has been slowwalking their decision on this very important matter, and we will need a definitive answer in a very short period of time!"

But optimism among both U.S. and Iranian officials appears to have cooled in recent days.

An Iranian official, who spoke with ABC News on the condition of anonymity to discuss sensitive negotiations, said that the U.S.' proposed terms are "unreasonable, greedy and unconventional."

"The U.S. constantly changes its positions, which has led to a growing accumulation of mistrust regarding its intentions and heightened uncertainty about its will and seriousness to fulfil commitments, assuming any agreement is even to be formed," they told ABC News.

"This document is not even open to be reviewed or responded to," the official added.

Iran is pushing for relief from sanctions that have badly damaged the national economy. This week, Iranian Foreign Ministry spokesperson Esmail Baghaei said Tehran needs guarantees regarding the "real end of the sanctions."

That would include details on "how and through what mechanism" they would be lifted, Baghaei said, in remarks carried by the official IRNA news agency.

Araghchi and Khamenei were downbeat on the U.S. proposal in comments this week.

The document has "many ambiguities and questions," the Iranian foreign minister said in a Telegram post on Tuesday. "Many issues in this proposal are not clear," he added.

-ABC News' Ellie Kaufman, Morgan Winsor, and Will Gretsky contributed to this report.