The US says it is deeply troubled by Iran's plans to start a reactor next year that could yield nuclear bomb material.
It also raised fears over the country's failure to give UN inspectors design information about the plant.
The comments by a US envoy to a board meeting of the UN's International Atomic Energy Agency (IAEA) highlighted western concern about the heavy water reactor that Iran is building near the town of Arak.
Tension over Iran's nuclear course is rising with talks between Tehran and six powers stalled.
Israel, widely assumed to be the Middle East's only nuclear-armed state, sees Iran as the most serious risk to it and has threatened to bomb its nuclear sites if diplomacy and sanctions fail to restrain Tehran.
The IAEA says Iran must give it design data about Arak, warning it would otherwise restrict its ability to monitor the site.
To signal unity on Iran, China and Russia joined four western powers in pressing it to cooperate with a stalled IAEA investigation into suspected atomic weapons research. However, like previous such diplomatic initiatives, it looked unlikely to have an impact in softening Iran's defiance.
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