Iran said on Thursday the United States has made extra demands and accused it of working to "complicate" efforts to restore a 2015 nuclear deal, after new Russian requests linked to the Ukraine war raised concerns of fresh delays.
Iran's supreme leader Ayatollah Ali Khamenei, who has the final say in major state policies, meanwhile, stressed his country will not give up on elements of "national strength", such as nuclear progress and regional influence.
Tehran is locked in negotiations with world powers to revive the nuclear deal that offered it sanctions relief in return for curbs on its atomic programme.
Its arch-foe the United States, under former president Donald Trump, unilaterally withdrew in 2018 from the accord known formally as the Joint Comprehensive Plan of Action.
"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.
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 coordinator", in reference to the EU.
Earlier on Thursday, Iran's top security official Ali Shamkhani tweeted that "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 US isn't interested in a strong deal that would satisfy both parties," he added.
- Russian factor -
The United States 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."
Moscow has said it wants written assurances from Washington that sanctions imposed on it over the Ukraine war will not affect its economic and military cooperation with Tehran.
The negotiations to revive the deal involve Iran as well as France, Germany, Britain, Russia and China directly, and the United States 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 new 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 Wednesday.
"Like any other participant we have the right to ask for something... It's normal business," he added.
- 'National strength' -
"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," Price told reporters.
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 program and regional influence and ties to armed groups from Lebanon to Yemen.
Iran's supreme leader said Thursday that 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?" Khamenei asked, 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?"