There was no indication late Thursday that the six powers were ready to go that way. But moderate reactions from some suggested they were eager to keep negotiation channels open.
The British Foreign Office confirmed that Iran had informed the International Atomic Energy Agency of its plan, and described it as "a cause for concern," noting it breached both U.N. Security Council and IAEA board resolutions urging Iran to curb enrichment.
But it avoided linking the move to the next round of talks. Instead the statement expressed hope that Iran would soon respond to the six powers on a time and place for a meeting, adding: "We hope that Iran will agree to talks quickly and come to the table ready to engage and negotiate seriously."
In Moscow, Russian Foreign Minister Sergei Lavrov noted Thursday that Moscow and its fellow U.N. Security Council members "have called on Iran to freeze enrichment operations during the negotiations." But he too avoided any direct suggestion that the planned Iranian centrifuge update would upend such talks.
The White House said the move by Iran did not come as a surprise, describing it as a further escalation and continuing violation of Iran's international obligations.
"It would mark yet another provocative step by Iran, and will only invite further isolation by the international community," said White House spokesman Jay Carney. "We continue to believe that there is time and space for diplomacy to work, but actions like this only undercut the efforts of the international community to resolve its concerns."
The European Union's top foreign policy official, Catherine Ashton, said she is confident negotiations over Iran's nuclear program will resume soon. Ashton has convened past meetings, and her spokesman had suggested last week that Iran was delaying by setting new preconditions and not agreeing to a venue.
A Western diplomat accredited to the U.N. agency said IAEA delegation heads from the U.S. and its allies exchanged views over Iran's plans Thursday and agreed to await further developments. He also demanded anonymity because he was not authorized to talk about the issue.
While acknowledging that the Iranian plan was of concern, he noted that Tehran had set no date for installing the new centrifuges. That, he said, gave the international community breathing space.
Iran says it is enriching only to power reactors and for scientific and medical purposes. But because of its nuclear secrecy, many countries fear that Iran may break out from its present production that is below the weapons-grade threshold and start enriching uranium to levels of over 90 percent, used to arm nuclear weapons.
Tehran now has more than 10,000 centrifuges enriching uranium at its main plant at Natanz, 225 kilometers (140 miles) southeast of Tehran, to fuel grade at below 4 percent. Its separate Fordo facility, southwest of Tehran, has close to 3,000 centrifuges — most of them active and producing material enriched to 20 percent, which can be turned into weapons-grade uranium much more quickly.
Iran has depended on domestically made and breakdown-prone IR-1 centrifuges whose design is decades-old at both locations up to now, but started testing more sophisticated prototypes in the summer of 2010.
David Albright, whose Washington-based Institute for Science and International Technology serves as a resource for some U.S. government branches, estimated in a 2011 report that 1,000 of the advanced machines "would be equivalent to about 4,000-5,000 IR-1 centrifuges" in production speed.
Separately from the talks between Iran and the six powers, IAEA experts are scheduled to visit Tehran on Feb. 13 in their more-than-yearlong effort to restart the probe of the weapons allegations.
The British statement urged Iran to "take serious practical steps to cooperate with the IAEA on all matters of substance relating to the possible military dimensions to its nuclear programme."