Talks are under way in high-level negotiations to limit Iran's nuclear programme in exchange for easing crippling sanctions on the Islamic republic's economy.
Facing a November 24 deadline before negotiations expire, US secretary of state John Kerry met Iranian foreign minister Javad Zarif and European Union adviser Catherine Ashton.
Mr Kerry has said real gaps between Tehran and world powers still exist in the negotiations. But the US remains hopeful a deal can be brokered in what would mark a major foreign policy success for the Obama administration.
The three diplomats did not make any comments to journalists at a brief photo opportunity as the negotiations began in Oman.
Negotiators from Britain, China, France, Germany and Russia are expected to join the final round of talks starting on November 18 in Vienna.