Senior Iranian and US officials held what Tehran's top negotiator dubbed "constructive" talks on Monday, as Washington warned tough choices were needed for a lasting accord on Tehran's controversial nuclear programme by a July 20 deadline.

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.