Over the last six months, Iran has evaded U.S. sanctions by importing Turkish gold to pay for billions of dollars worth of energy sales to Turkey.

Turkey's deputy prime minister has described what amounts to a gold-for-oil barter system.

"Why did, all of the sudden, Turkey's gold exports, especially gold bullion, go up?" Ali Babacan asked while speaking before a parliamentary budget commission this month. The official transcript of his statements was published by a Turkish government website Wednesday.

Since the sanctions have been unilaterally imposed by Washington, Turkey is not breaking any international law by trading oil for gold with Iran. But in doing so, the Turks risk incurring the wrath of the U.S. government, an important military and political ally.

"At the end of July, the President issued an order that authorizes Treasury to impose sanctions on anyone who helps the Government of Iran acquire U.S. dollars or precious metals, including gold," a U.S. Treasury Department spokesman wrote to CNN this week, on condition he not be named. "We can't comment on any investigations that may be ongoing."

Economists say the gold-for-oil trade shows the sanctions are hurting Iran.