Britain expressed "serious concern" on Thursday at news that Iran had installed new nuclear equipment, saying it was another sign of Tehran's refusal to address Western fears about its nuclear programme.
Five days before international talks between Iran and six world powers, the International Atomic Energy Agency (IAEA) said in a new report that Tehran has begun installing next-generation equipment at one of its main nuclear plants.
"Iran's installation of more advanced centrifuges at its enrichment plant at Natanz is of serious concern," said Foreign Office minister Alistair Burt.
"It represents yet another breach of United Nations Security Council and IAEA Board resolutions, and is another signal that Iran has no intention of providing the necessary reassurance to the international community that its nuclear programme is for purely peaceful purposes."
He added: "The timing of this move -- immediately before the IAEA board of governors meeting and before E3+3 talks with Iran in Almaty, Kazakstan next week -- concerns and disappoints us.
"We trust that Iran will come to talks in Almaty willing to negotiate meaningfully, and we urge Iran to respect all relevant resolutions and cooperate fully with the IAEA."
Iran denies seeking atomic weapons but many in the international community suspect otherwise, and the UN Security Council has passed several resolutions calling on Iran to suspend all uranium enrichment.
Iran and six world powers -- the US, China, Russia, Britain, France and Germany -- are due to hold talks in Kazakhstan on February 26, the first such discussions since three rounds of meetings last year in Moscow ended in stalemate.