The fourth round of talks between Iranian and US nuclear delegations started in Montreux, Stwitzerland, on Wednesday, IRNA news agency reported.
The delegations are led for Iran by Foreign Minister Mohammad Javad Zarif and for the US by Secretary of State John Kerry.
Zarif and Kerry held nuclear discussions on Tuesday morning here as well.
Head of Iran's Atomic Energy Organization Ali Akbar Salehi and US Energy Secretary Ernest Moniz are also attending the discussions.
Iran and the United States talked for two and a half hours on Monday evening, too.
Deputy foreign ministers of Iran and the US accompanied by deputy EU foreign policy chief resumed nuclear talks in Montreux earlier Monday.
Iranian deputy foreign ministers Abbas Araqchi and Majid Takht-e Ravanchi, their US counterpart Wendy Sherman and the EU foreign policy deputy Helga Schmid are seeking to narrow the gap as Iran and the six world governments are aiming to reach a political understanding by the end of March in the run-up to a comprehensive deal by a self-imposed June 30 deadline.
Foreign ministry deputies of other countries involved in the negotiations are to join the talks here on Thursday.