The value of the US dollar has fallen steadily in recent times

The call came at the end of a rare Opec summit, and was opposed by US ally Saudi Arabia.

The Iranian president had wanted to include the attack on the dollar in the summit's closing statement.

The communique made little mention of the dollar, however, focusing instead on energy security and the environment.

The summit in Saudi Arabia was only Opec's third in 47 years.

During the talks, Opec members revealed differences about the future direction of the exporters' group.

But Opec leaders ended with a pledge to provide the world with reliable supplies of oil.

Unfair trade?

Speaking after the end of the summit, Mr Ahmadinejad said all leaders at the meeting were unhappy with recent falls in the value of the dollar.

Oil reached a record high of $98.62 a barrel earlier this month

Its decline has affected the revenues of Opec members because most of them price and sell their oil exports in the US currency.

Mr Ahmadinejad said that all Opec countries had showed interest in converting their cash reserves into other currencies.

"They [the US] get our oil and give us a worthless piece of paper," he told reporters.

But Saudi officials were against including any such language in the declaration. One is reported to have warned that it could add to the pressure on the dollar.

However, in the communique Opec did make a reference to the debate, by committing itself to studying "ways and means of enhancing financial co-operation".

Iran's oil minister said that this would allow the formation of a committee to study the dollar's effect on oil prices and investigate the possibility of alternative trading currencies.

Political agenda

The summit was also marked by divisions over the role of Opec in the world oil market.

Chavez (r) and Iran's Mahmoud Ahmadinejad are staunch US critics

King Abdullah, the head of state of the host nation, Saudi Arabia said: "Those who want Opec to take advantage of its position are forgetting that Opec has always acted moderately and wisely.

"Oil shouldn't be a tool for conflict, it should be a tool for development."

President Chavez had opened the meeting with a warning that oil prices could double if the US attacked Iran.

Oil has been hitting record peaks of well over $90 a barrel as markets believe the Organisation of Petroleum Exporting Countries will not boost production, despite calls from oil-consuming countries such as the US to do so.

Venezuela's president said the price of crude could reach $150 or even $200 a barrel.