U.S. Deputy Secretary of State Bill Burns addresses a news conference in Tunis

U.S. Deputy Secretary of State Bill Burns addresses a news conference in Tunis February 1, 2014.

U.S. Deputy Secretary of State Bill Burns addresses a news conference in Tunis February 1, 2014. REUTERS/Zoubeir Souiss

U.S. No. 2 diplomat expected to attend Iran nuclear talks in Vienna