The latest monthly report by OPEC says Iran's oil output in June has dropped by 142,000 barrels per day compared to the previous month, reaching almost 2.23 million barrels.
Iran's output was 3.8 million barrels per day before the return of US sanctions.
Meanwhile, estimates show that Iran's oil exports have dropped below 300,000 barrels per day. The figure was 2.5 million barrels per day before the sanctions.
The United States withdrew from the nuclear deal with Iran and ended sanctions waivers for Iran's customers in May as part of the economic pressures on Iran to agree to a new deal with the West.
The OPEC report puts the cartel's overall output in June at 29.83 million barrels. The figure was over 32 million barrels in 2017.
The decline in OPEC's overall output is because of the drop in Iranian and Venezuelan oil production.