U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Jawad Zarif have held two rounds of talks on Tehran's disputed nuclear program.
The two leaders met for five hours before another unscheduled meeting in the evening on January 14 in Geneva.
A U.S. State Department official described the discussions as "substantive."
Zarif had said the meeting would help gauge whether both sides were ready to advance toward a deal to curb Iran's nuclear program in exchange for sanctions relief.
Zarif and Kerry met a day before U.S. and Iranian delegations were to meet in Geneva.
Wendy Sherman, U.S. acting deputy secretary of state, and Abbas Araqchi, Iranian deputy foreign minister, are to lead their delegations on January 15 for discussions expected to focus on the technical issues of the negotiations.
That is to be followed on January 18 by wider talks between Iran and six world powers.
In 2013, the United States, France, Germany, Britain, Russia, and China -- the so-called P5+1 -- struck a preliminary agreement with Iran to suspend nuclear work in exchange for easing some economic sanctions.
Iran denies it is secretly developing nuclear weapons, as the West suspects.
Iran and the P5+1 have missed two self-imposed deadlines to reach a final, comprehensive deal.
The main stumbling block is reported to be Iran's insistence it be able to enrich some uranium -- which can in some cases be used to make a nuclear bomb -- for what it says is a peaceful civilian nuclear program.
The two sides are now facing a June 30 deadline to secure a deal, although some analysts say reaching an agreement could become even more elusive.
The new Republican-controlled Congress in the United States is considering a bill that would slap new sanctions on Iran, despite objections from the Obama administration.
U.S. Ambassador to the UN Samantha Power warned on January 12 that "imposing new sanctions will almost certainly end a negotiations process that has not only frozen the advance of Iran's nuclear program, but that could lead us to an understanding that would give us confidence in its exclusively peaceful nature."
With reporting by Reuters and AP