US brands Iran centrifuges 'provocative'
21 February 2013 Last updated at 21:23 GMT
Iran is installing advanced centrifuges at a uranium enrichment plant in Natanz, the UN nuclear watchdog has said.
Speaking in Washington, DC state department spokeswoman Victoria Nuland said the move represented "yet another provocative step" by Iran.
But, she added, there was another path forward at forthcoming talks, saying she hoped Iran would consider taking the "diplomatic path".
Western powers fear Tehran is seeking weapons technology, but Tehran says it is refining uranium only for peaceful energy purposes.