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."

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.

"They get our oil and give us a worthless piece of paper,"...