"While significantly higher production is unlikely before next year, oil held in floating storage - at the highest level since sanctions were tightened in mid-2012 - could start to reach international markets before then," the IEA said in a monthly report.
Iranian Oil Minister Bijan Zanganeh has said Iran expects to raise oil output by 500,000 bpd as soon as sanctions are lifted and by a million bpd within months.
The report by the Paris-based IEA suggested any increase in output would probably be more modest than Iranian estimates, and said the Islamic Republic would require massive investment to raise output capacity.
Iran has said it hopes to secure nearly $200 billion worth of oil and gas projects with foreign partners by 2020.
Iran and six world powers agreed a deal in July aimed at curbing Tehran`s nuclear programme. However, sanctions will not be lifted until Iran has complied with all the terms of the pact, and the agreement has to be ratified by the U.S. Congress.
Sanctions are not expected to be removed before the end of December and analysts say they may still be in place at the end of the first quarter of 2016.
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