But after a meeting of Iran's foreign minister and world powers, U.S. Secretary of State John Kerry was still skeptical.
"Needless to say one meeting and a change in tone, which was welcome, doesn't answer those questions yet and there is a lot of work to be done," said Kerry.
Iran's foreign minister called the same meeting constructive while sticking to Tehran's bottom line:
"As we move forward, there has to be removal of sanctions, and, in the end game, there has to be a total lifting of all sanctions," said Javad Zarif.