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 nuclear watchdog, would be a "provocative step."
The move "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 added.
The International Atomic Energy Agency earlier said Iran started installing the new IR-2m centrifuges 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.
Nuclear enrichment is at the heart of the dispute over Tehran's disputed program, which the West and Israel say is a front to develop nuclear weapons.
Iran insists its program is strictly for peaceful purposes.
White House spokesman Jay Carney warned Tehran it had a "choice."
"If it fails to address the concerns of the international community, it will face more pressure and become increasingly isolated," he said.
"The burden of sanctions could be eased, but the onus is on Iran to turn its stated readiness to negotiate, into tangible action."
The IAEA 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.
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.
Carney insisted the P5+1 was "united" in its approach on Iran.
"We simply call on the Iranians to arrive at those talks with the intention of having them be substantive and focused on the issues that are of concern here to the international community," he added.
Nuland, who said the report of new centrifuges was 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.