Iran is expected to add half a million barrels of oil supply a day within a year from its existing oilfields after the lifting of sanctions against Tehran in January, but developing new fields would take time, the head of the International Energy Agency said on Wednesday.

Iran, previously OPEC's second-largest exporter, would need to prove that the investment conditions were profitable to the international investors and also that there was predictability in the markets, Fatih Birol, IEA's executive director told Reuters.

Birol's estimate of Iran's supply increase from existing oilfields was in line with previous market estimates.

And increases in Iranian gas supplies would come after oil, he said.

"It was misleading to believe that there would be a huge amount of new Iranian crude and natural gas production entering market in the short term," Birol said on the sidelines of an event in Beijing to mark the 20th anniversary of cooperation between China and IEA.

"It would take some time in terms of developing new oil fields, finding transmission routes and having the necessary market conditions."

Iranian oil officials were hoping for a quick rebound in oil sales to European clients, which accounted for over a third of Iran's exports, or 800,000 barrels per day, before the European Union imposed sanctions in 2012 over Tehran's nuclear programme.

The IEA chief said that may take some time too given ample supply in the market and lack of growth prospects in Europe.