Iranian Foreign Minister Mohammad Javad Zarif and US Secretary of State John Kerry met here late on Saturday, IRNA reported.
It is expected that their discussions will drag beyond midnight.
Earlier in the day, Zarif met French Foreign Minister Laurent Fabius and special envoy of Russian foreign minister.
In the meantime, representatives from P5+1 group of world powers and EU convened as negotiators are approaching the newly set deadline of July 13.
Zarif's deputies also held talks with EU and US envoys separately.