The United States warned Iran Thursday that the installation of next-generation centrifuges at one of its main nuclear plants, as reported by the UN atomic agency, would be a "provocative step."
The installation "would be a further escalation and a continuing violation of Iran's obligations under the relevant UN Security Council resolutions and IAEA board resolutions," State Department spokeswoman Victoria Nuland said.
"So it would mark yet another provocative step," she said.
The International Atomic Energy Agency, the UN nuclear watchdog, on Thursday said Tehran had begun installing the new IR-2m centrifuges earlier this month at the Natanz plant.
"This is the first time that centrifuges more advanced than the IR-1 have been installed" at the plant in central Iran, an IAEA report said.
The report comes five days before Iran is set to sit down with world powers in Kazakhstan for the latest round of talks on the Islamic republic's nuclear program, which the West and Israel say is a front for weapons development.
Tehran insists its program is strictly for peaceful purposes.
The talks between the so-called P5+1 -- the United States, China, Russia, Britain, France and Germany -- and Iran will be the first since June, when three rounds of meetings ended in stalemate in Moscow.
Nuland, who said the report of new centrifuges were no surprise, urged Iran to consider "another path... the diplomatic path."
"They have an opportunity to come to those talks ready to be serious, ready to allay the international community's concerns, and we hope they take that opportunity," she said.