Tense talks in Vienna between Iran and world powers to revive the 2015 nuclear deal and remove US sanctions have hit another roadblock as Russia has “complicated” the situation by requesting the deal be linked to the Ukraine war.
Iran, meanwhile, suggested there were new hurdles to clear, while Washington said hard issues remained.
Iran’s supreme leader, Ayatollah Ali Khamenei, who has the final say in major state policies, said his country would not give up on elements of "national strength", such as nuclear progress and regional influence.
Iran is in discussions with the US and world powers to revive the 2015 nuclear deal – known as the Joint Comprehensive Plan of Action – that former American president Donald Trump walked out on in 2018 and imposed crippling sanctions on Tehran.
"There is no rational justification for some of the new demands made by the United States, and it contradicts the country's position on reaching an agreement swiftly," Iranian Foreign Minister Hossein Amir-Abdollahian said in a phone call with EU foreign policy chief Josep Borrell, his ministry reported.
Mr Amir-Abdollahian did not specify the demands but said "the US cannot pass on a new and different message to us every day through the co-ordinator", in reference to the EU.
Earlier on Thursday, head of Iranian security Ali Shamkhani tweeted: "Vienna negotiations are becoming more complicated every hour without a political decision by the United States.
"US approach to Iran's principled demands, coupled with its unreasonable offers and unjustified pressure to hastily reach an agreement, show that the US isn't interested in a strong deal that would satisfy both parties."
At the same time, however, Iran also offered an opening.
"Efforts to reach a good and durable agreement continue; it is within reach if US acts realistically and consistently," Mr Amir-Abdollahian said on Twitter.
The US reaffirmed its position that a deal remained close and could even be reached "in the coming days".
"It's really down to a very small number of outstanding issues," State Department spokesman Ned Price said at a news conference on Thursday, without mentioning any new demands from Washington.
"But the reason these particular issues are outstanding is because they are among the most difficult ones."
Separately, Moscow has said it wants written assurances from Washington that sanctions imposed over the Ukraine war would not affect its economic and military co-operation with Tehran.
The US has ruled out such a move.
"We also have no intention of offering Russia anything new or specific as it relates to sanctions, nor is anything new required to successfully reach an agreement on a mutual return to full compliance with the JCPOA," Mr Price said.
The negotiations to revive the deal involve Iran as well as France, Germany, Britain, Russia and China directly, and the US indirectly.
Washington's unilateral withdrawal from the accord and its reimposition of biting economic sanctions prompted Iran to begin rolling back on its own commitments.
The United States has described Russia's latest demands as "irrelevant", while France warned they could dash hopes for a revived nuclear accord.
"Some people are trying to blame us for protracting the talks. I must tell you that the talks have not yet been finalised, even the text of a final agreement is not yet finalised," Russian chief negotiator Mikhayil Ulyanov said.
"Like any other participant, we have the right to ask for something ... it's normal business."
But Mr Price told reporters: "We've urged all parties – and of course that includes the Russian Federation – to focus on resolving the final remaining issues so that we can achieve our shared objective that is an Iran that is permanently and verifiably barred from ever obtaining a nuclear weapon."
He said the Biden administration had made clear it had "no intention of offering Russia anything new or specific as it relates to the sanctions nor is anything new required to successfully reach an agreement on a mutual return to full compliance."
The July 2015 deal gave Iran sanctions relief in exchange for curbs on its nuclear activities to guarantee it could not develop a weapons capability – an ambition it has staunchly denied.
Western countries have also voiced concern over other Iranian capabilities, including its ballistic missiles programme, regional influence and ties to armed groups from Lebanon to Yemen.
Mr Khamenei said matters of "national strength" were not up for negotiation.
"Regional presence gives us strategic depth and more national strength. Why should we give it up?" he said in a statement on his official website.
"Nuclear scientific progress is also related to meeting the needs of the country in the near future, and if we give it up, from who and where we should ask for that in a few years?"