The Islamic republic is locked in negotiations with world powers to revive the 2015 deal that offered it sanctions relief in return for curbs on its nuclear 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.

Moscow said on Saturday that, before backing a revived deal, 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.

"Vienna negotiations are becoming more complicated every hour without a political decision by the United States," Iran's top security official Ali Shamkhani tweeted on Thursday.

"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.

The negotiations to revive the deal involve Iran as well as France, Germany, Britain, Russia and China directly, and the United States indirectly.

The US's unilateral withdrawal from the accord and its reimpositon of biting economic sanctions prompted Iran to begin rolling back on its own commitments.

Negotiators had recently signalled that talks had progressed into a "final phase", but pending issues were still unresolved.

The United States has described Russia's new demands "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 that the talks have not yet been finalised, even the text of final agreement is not yet finalised," Russian chief negotiator Mikhayil Ulyanov said on Wednesday.

"Like any other participant we have the right to ask for something... it's normal business, those who don't understand it are not professionals." he added.

"We have the right to protect our interest both in the nuclear field, as well as in the wider context," he said, adding that he believes "all our trade and economic relations with Iran should be exempt from current and future EU or US sanctions".