Alcoa posted its worst day since 2009 after the U.S. Treasury Department issued a six-month reprieve on sanctions levied against a top Russian aluminum competitor.

Its stock fell more than 13.5 percent after the department said it would give Americans until October to wind down transactions with Rusal, the stock's worst performance in a single day since March 30, 2009. The extension was granted, Treasury Secretary Steven Mnuchin said, to give the department time to review its penalties on the company.

Aluminum prices also fell.

"Rusal has felt the impact of U.S. sanctions because of its entanglement with Oleg Deripaska, but the U.S. government is not targeting the hardworking people who depend on Rusal and its subsidiaries," Mnuchin said in a statement Monday.