There has been planning for the presence of the foreign ministers of the sextet (in the nuclear negotiations) but nothing is certain, Iranian Deputy Foreign Minister Seyed Abbas Araqchi said on Thursday.
According to some media reports, Russian Foreign Minister Sergey Lavrov is planned to join the talk on Sunday.
Diplomats from Iran and the Group 5+1 (also known as the P5+1 or E3+3) will start the second day of the new round of negotiations in Lausanne on Friday.
On the first day, Iranian Foreign Minister Mohammad Javad Zarif and his US counterpart John Kerry held separate meetings and exchanged views on the nuclear issue.
Head of the Atomic Energy Organization of Iran (AEOI) Ali Akbar Salehi also held talks with US Energy Secretary Ernest Moniz in Lausanne on Thursday.
In the meantime, Araqchi and US Undersecretary of State Wendy Sherman, and Deputy Secretary General for the European Union's External Action Service Helga Schmid held a trilateral meeting in the Swiss city.
Iran and the sextet are in talks to hammer out a comprehensive accord by a June 30 deadline that would end more than a decade of impasse over Tehran's civilian nuclear activities.
The negotiations came after diplomats from Iran and the Group 5+1 wrapped up six days of negotiations in Lausanne on March 20.