In last-ditch talks to stop the sanctions, European powers have signalled they could relent if Iran returns to the negotiating table with the US. Several such meetings were held in spring, before Israel and the US bombed Iran during a 12-day war in June.
Mr Khamenei – who was in danger of being assassinated by Israel during the aerial war – said the US had effectively "announced the result of the talks in advance" by saying Iran should not be able to enrich uranium at all.
Iran insists its enrichment programme is for peaceful purposes only. But western powers say the enrichment comes worryingly close to the level needed for a nuclear bomb.
Negotiating with the US "not only brings no benefit, but also causes major harm under the current conditions, some of which might even be described as irreparable", Mr Khamenei said in his televised address.
Talks with the US represent “a sheer dead end”, Mr Khamenei said. His comments are likely to limit any possible outreach by Iranian President Masoud Pezeshkian while he attends the UN General Assembly in New York.
Britain, France and Germany initiated the move to restore sanctions this month, saying Iran had flouted the terms it agreed to under the deal by amassing a uranium stockpile more than 40 times the limit.
Iran largely admits to this but says US President Donald Trump's withdrawal from the pact in 2018 effectively made the deal null and void. Talks with European powers have failed to avert the so-called sanctions snapback.
The German Foreign Office said Britain, France and Germany had urged Iran to take practical steps within the days, "if not hours", to address concerns around its nuclear programme.
"We are prepared to complete the snapback process if necessary, but that will not mark the end of our engagement. We will remain committed to finding a diplomatic solution to Iran’s nuclear programme," the Foreign Office said in a statement on X.
What damage the US bombing inflicted on Iran's nuclear stockpile remains unclear. Egypt recently brokered a deal under which UN inspectors could return to Iran but details are vague.
