Talks on Iran's nuclear program have begun in Kazakhstan among the five permanent members of the U.N. Security Council, plus Germany and Iran.
On Thursday, Iran's chief nuclear negotiator, Saeed Jalili, demanded that world powers recognize an Iranian right to enrich uranium.
The United States and its allies accuse Iran of amassing highly-enriched uranium as part of a covert drive to make nuclear bombs. Tehran insists its nuclear program is peaceful.
A senior U.S. official said Wednesday the world powers want Iran to give a "concrete and substantive" response to their proposal for cutting enrichment activities in return for an easing of international sanctions. The six nations presented that offer to Iran at the last round of negotiations in Almaty in late February.
The U.S. official reiterated that President Barack Obama prefers diplomacy, but is leaving "all options" on the table to deal with the situation.