The United States reacted with concern Wednesday after Iran unveiled a new uranium production facility and two extraction mines, but said it had not been "blindsided" by the news.
Iranian President Mahmoud Ahmadinejad hailed the advances on Tuesday and boasted of mastery over "the entire chain of nuclear energy" only days after talks with world powers on its disputed nuclear programme ended in deadlock.
"They have continued to move forward, we are very concerned about what they are doing," a senior State Department official said, asking to remain anonymous.
The official acknowledged that despite intensive and "substantive" discussions in Almaty, Kazakhstan last week, Iran had given no hint of the news.
"We weren't blindsided about it, because we are rarely blindsided about the things that they are considering. But they did not specify that they were going to do this," the official said.
"They did specify that they were going to proceed forward and do everything they could to fulfil what they see as their inalienable right," the official told reporters travelling with US Secretary of State John Kerry.
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Kerry on Tuesday denounced the Iranian news as "provocative."
"To make any step that increases the rapidity with which you move towards enriched fissile material raises the potential of questions, if not even threat," he told reporters at the end of a visit to Israel.
"The clock that is ticking on Iran's programme has a stop moment and it does not tick interminably."
The United States and its partners in the P5+1 talks wanted a diplomatic way forward, Kerry stressed, but "negotiations are not for the sake of negotiations, they are to make progress."
"Negotiations cannot be allowed to become a process of delay which in and of itself creates greater danger."
Iran's nuclear programme will be discussed at a meeting of G8 foreign ministers in London on Wednesday and Thursday.
And the US official praised the international sanctions, aimed at choking off supplies and cash for Iran's nuclear program, which have been hitting exports of Iranian oil abroad -- a key currency earner for the regime.