Foreign Minister Abbas Araghchi said Wednesday that Iran's enrichment of uranium as part of its nuclear programme was "non-negotiable" after US Special Envoy Steve Witkoff called for a halt.
"Iran's enrichment is a real, accepted matter. We are ready to build confidence in response to possible concerns, but the issue of enrichment is non-negotiable," Araghchi told reporters after a cabinet meeting.
The remarks came as Araghchi and Witkoff are due to meet again in Oman on Saturday, a week after they held the highest-level talks between the longtime foes since US President Donald Trump abandoned a landmark nuclear deal in 2018.
Trump reimposed sweeping sanctions in a policy of "maximum pressure" against Tehran that he has reinstated since returning to office in January.
In March, he sent a letter to Iran's supreme leader Ayatollah Ali Khamenei urging talks but warning of possible military action if they fail to produce a deal.
Both sides described Saturday's meeting as "constructive".
But on Tuesday, Witkoff said Iran must "stop and eliminate" its enrichment of uranium as part of any nuclear deal.
He had previously demanded only that Iran return to the 3.67 percent enrichment ceiling set by the 2015 accord between Iran and major powers that Trump withdrew from.
Araghchi condemned what he called the "contradictory and conflicting positions" coming out of the Trump administration ahead of Saturday's talks.
"We will find out the true opinions of the Americans during the negotiation session," he said.
Iran's top diplomat said he hoped to start negotiations on the framework of a possible agreement but said that required "constructive positions" from the United States.
"If we continue to (hear) contradictory and conflicting positions, we are going to have problems," he warned.
On Tuesday, Khamenei cautioned that while the talks have proceeded well in their early stages, they could still prove fruitless.
"The negotiations may or may not yield results," he said.
Kremlin says ready to help
The Kremlin said Wednesday that Russia was ready to do "everything" in its powers to help find a diplomatic resolution to the stand-off between the United States and Iran over Tehran's nuclear programme.
Moscow, which commands the world's largest confirmed arsenal of nuclear weapons, has deepened its military ties with Iran since it launched its offensive on Ukraine in February 2022.
"The Russian Federation remains ready to do everything within our capabilities to contribute to the settlement of the situation by political and diplomatic means," Kremlin spokesman Dmitry Peskov told reporters. He was responding to a question from AFP over whether Moscow would consider being a guarantor of any deal.
An Iranian MP had raised the possibility of Russia and China being joint guarantors of any future agreement in an interview with Russia's state-run RIA Novosti.
Russia has issued multiple calls for calm after US President Donald Trump last month appeared to threaten to bomb Iran if it did not engage in talks about restricting its nuclear programme.
Peskov did not say whether Russia would act as guarantor to any agreement, only that Moscow was "counting" on a second round of talks between Iran and the United States.
Iranian and US officials held indirect talks in Oman last week, the highest-level meeting between the two sides since Trump abandoned a landmark nuclear deal in 2018.