Negotiations over Iran's nuclear program are underway in Vienna. But reports of multibillion-dollar oil talks between Tehran and Russia are posing another obstacle to reaching an agreement.
A Russian newspaper reports Moscow plans to buy 500,000 barrels of Iranian oil a day, violating an export limit under the deal reached in Geneva last November.
The oil deal could provide Tehran with billions of dollars, undermining the impact of years of U.S. and international sanctions that have crippled Iran's economy.
The United States is considering potentially deal-breaking sanctions if a contract is completed.