ISTANBUL (Reuters) - Shares in Turkey's Halkbank tumbled as much as 16 percent on Wednesday, after U.S. prosecutors charged one of its senior executives with participating in a multi-year scheme to violate U.S. sanctions against Iran.

At 0716 GMT, Halkbank <HALKB.IS> shares were 11.28 percent lower. Istanbul's banking index <.XBANK> was down 2.22 percent and the main index <.XU100> was 0.98 percent lower.

(Writing by Daren Butler; Writing by David Dolan)