The closed-door meeting in Geneva, due to last two days, marks a new effort to find common ground between Tehran and Washington, amid concerns that tensions between the two could damage efforts to strike deal between the Islamic republic and world powers.
As the first day of talks drew to a close, Washington said time was running out.
"We think we've made progress during some rounds, but as we said coming out of the last one, we hadn't seen enough made. We hadn't seen enough realism, quite frankly, on the table," said deputy State Department spokeswoman Marie Harf.
"People need to make tough choices, but we are very focused on that July 20th time."
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Iran's deputy foreign minister and nuclear pointman Abbas Araqchi said the dialogue "took place in a positive climate and was constructive," in comments carried by Iran's ISNA news agency.
The Geneva meeting marks the first time since the 1980s that Tehran and Washington have held official, direct talks on the nuclear issue outside of the P5+1 process.
For Iran, the goal is to make a leap towards ending the international sanctions that have battered its economy.
Time is running out for Iran's negotiations with the so-called P5+1 group, which includes the five permanent UN Security Council members Britain, China, France, Russia and the United States, plus Germany.
A deadline of July 20 has been set to turn a temporary deal struck in November in Geneva into a permanent agreement.
"If this does not happen, we'll have to resort to extending the Geneva agreement for another six months so the negotiations can continue," Iran's IRNA news agency quoted Araqchi as saying earlier today.