Iranian officials began talks at 13:00 local time on Saturday at the Serena Hotel in Islamabad, Pakistan, first with Pakistani officials and then with the American delegation.
The marathon of talks continued until 3:40 am local time on Sunday.
The talks are scheduled to continue for a few more hours on Sunday.
Tasnim reporter dispatched to Islamabad reported that “given the unreasonable demands made by the American side and the Iranian delegation's insistence on preserving national interests, with the proposal of the Pakistani side and the acceptance of both sides, the talks will continue for another round on Sunday.
Earlier, Tasnim correspondent had reported that another round of face-to-face talks between Iran and the United States ended in Islamabad, Pakistan on Sunday morning.
The two sides once again exchanged texts with the presence of expert teams.
That came as some serious differences between the Iranian and American delegations in the talks still remain.
The ball is in America's court to overcome its recurring excessive demands and replace ambition with a realistic approach.