An Iranian official, briefed about the talks, told Reuters that the experts-level negotiations "are difficult, complicated and serious", without elaborating.

Leaders from the United States and Iran painted a positive but cautious picture of talks between the nations over Iran’s nuclear program Saturday, after the third phase of the discussions wrapped up in Oman.

The talks seek to limit Iran's nuclear program in exchange for the lifting of some of the crushing economic sanctions the U.S. has imposed on the Islamic Republic.