Iran is owed $4 billion for oil sales to customers who have been unable to pay because of sanctions imposed on the Islamic republic over its nuclear programme, a top Iranian oil official said on Sunday.

"We have been unable to get paid around four billion dollars due to the sanctions," National Iranian Oil Company chief Ahmad Qalebani told reporters when asked about the amount owed by customers, mostly Western, to Iran.

"There is a possibility that it could be paid either as medicine, food or barter of commodities," he told a press conference on the sidelines of an oil and gas trade fair in Tehran.

Qalebani did not give details on these customers.

But on Saturday, Oil Minister Rostam Qasemi said that Anglo-Dutch energy giant Shell was among the firms that owes Tehran petro-dollars which the Islamic republic can not repatriate due to sanctions.

"Currently, we have approximately $2.336 million payable to and $11 million receivable from National Iranian Oil Company. We are unable to settle the payable position as a result of applicable sanctions," Shell said in its 2012 annual report.

Last December, Economy Minister Shamseddin Hosseini said that Tehran is losing half of its oil revenues because of international sanctions imposed over its disputed nuclear programme.

Iran is struggling against what it calls an "economic war" to cope with punitive measures targeting its vital oil income and access to global financial systems.

An oil embargo imposed by the European Union on Iran came into effect in July 2012, ending European purchases of Iranian crude. It has also lowered purchases by major Asian customers under pressure from the United States.

Iran's oil output dropped to 2.67 million barrels per day in February from 2.72 in the previous month, OPEC said in April, citing secondary sources.

On Saturday, Qasemi confirmed that production and export of oil had declined in 2012.

"Our export has declined (in 2012) compared to the previous year because the European nations are not buying from us and naturally we have had a decline in oil production."

Iran is now OPEC's fourth biggest producer, after Saudi Arabia, Iraq and Kuwait, according to the cartel's data. In 2011, it ranked second.