WASHINGTON, Dec 13 (Reuters) - U.S. Republican President-elect Donald Trump on Tuesday formally announced Exxon-Mobil Corp chairman and CEO Rex Tillerson as his nomination to serve as U.S. secretary of state.

"His tenacity, broad experience and deep understanding of geopolitics make him an excellent choice for Secretary of State. He will promote regional stability and focus on the core national security interests of the United States," Trump said in a statement released by his transition team.

(Reporting by Susan Heavey; editing by John Stonestreet)