Secretary of State John Kerry says he is hopeful the U.S. and group of five other world powers can reach a deal with Iran on the country's nuclear program, but that he will not make any predictions.
Negotiators from Iran and the so-called P5+1 group that includes the U.S., Britain, China, France, Russia and Germany are meeting again Tuesday in Vienna.
They have less than a week to reach an agreement before their November 24 deadline.
Speaking Tuesday alongside British Foreign Secretary Philip Hammond, Kerry called this week's talks "critical."
"It's imperative, obviously, that Iran work with us in all possible effort to prove to the world the program is peaceful," he said.
Iranian media quoted Foreign Minister Javad Zarif saying Tuesday that a deal is still possible and expressing caution toward the international side to not make "excessive demands" of Iran.
The two sides have been negotiating for a year trying to agree on a comprehensive pact that ensures Iran's nuclear program is peaceful in exchange for a repeal of sanctions that have hurt the country's economy.
An interim agreement last November called for Iran to curb its most sensitive nuclear activity while receiving targeted sanction relief. The deadline for a wider deal was originally set for July, but with no agreement in place negotiators decided to continue the process for another six months.