Delegates from Iran and six world powers are meeting in Vienna to work out a deal aimed at curbing Iran's nuclear program in exchange for lifting U.N. and Western sanctions.
The parties face a July 20 deadline for a final agreement or risk the possibility that the talks could be extended and get tougher. Last month's negotiations ended in a stalemate.
A U.S. State Department spokeswoman says the talks are at a crucial stage with the parties still far apart.
Iran agreed last November to curb its uranium enrichment program, which the United States believes could be used to build a nuclear weapon. In exchange, the West agreed to ease sanctions on Iran and hold talks to establish a comprehensive deal.
Iran insists its nuclear program is strictly for peaceful civilian purposes.
Along with Iran and the United States, other participants in the talks are Britain, China, France, Germany, and Russia.
Some information for this report comes from AFP and Reuters.