There was no announcement of the sale on the bank's website and officials at the bank declined comment.
Following the report, the euro eased slightly versus the dollar from around $1.2227 to $1.2213.
Traders said the news was having limited impact on the euro, and some in the market were sceptical about whether Tehran would be able to buy significant amounts of dollars, given that U.S. depository institutions are banned from processing transfers involving the country.
Last month Iran's central bank governor hinted at a move away from the euro, reversing several years of Tehran publicly shunning the U.S. currency in keeping with its political hostility towards Washington.
Considering that the value of the euro is plummeting and that of the dollar is increasing, we will be examining the issue of changing the composition of Iran's hard currency basket, Mahmoud Bahmani was quoted as saying in the May 18 edition of Hezbollah daily. [ID:nLDE64H0B3]
Iran does not usually disclose the value of its reserves, though state television reported last December they exceeded $80 billion.
The report on the sale broke after official sources told Reuters some of the world's richest central banks would continue investing in Europe's ailing single currency, supporting its reserve status despite the sovereign debt crisis plaguing the region.
The liquidity of both the euro and the dollar and the difficulty of switching large portfolios to other currencies mean there are no alternatives to them in the near term, the sources in Brazil, India, Japan and South Korea said in separate interviews. [ID:nnLDE6510EO]
Last November, Bahmani said Iran's then policy of moving away from the dollar, both in its reserves and in the currency it received for its oil exports, had been beneficial. [ID:nDAH340738]
At a 2007 OPEC summit, Iran -- the world's fifth-largest oil exporter -- suggested oil should be priced in a basket of currencies rather than dollars, but it failed to win over other member states except Venezuela.
Ahmadinejad told reporters at that time the dollar was a worthless piece of paper.
(Writing by Robin Pomeroy; editing by John Stonestreet)