Saudi Arabia ? Iranian President Mahmoud Ahmadinejad said Sunday that OPEC’s members have expressed interest in converting their cash reserves into a currency other than the depreciating U.S. dollar, which he called a “worthless piece of paper.”

His comments at the end of a rare summit of OPEC heads of state exposed fissures within the 13-member cartel – especially after U.S. ally Saudi Arabia was reluctant to mention concerns about the falling dollar in the summit’s final declaration.

The hardline Iranian leader’s comments also highlighted the growing challenge that Saudi Arabia, the world’s largest oil producer, faces from Iran and its ally Venezuela within the Organization of Petroleum Exporting Countries.

Oil is priced in U.S. dollars on the world market, and the currency’s depreciation has concerned oil producers because it has contributed to rising crude prices and has eroded the value of their dollar reserves.