Motive behind attack on U.S. ambassador unclear: State Department

WASHINGTON (Reuters) - The motive behind an attack on U.S. ambassador to South Korea Mark Lippert is unknown, the U.S. State Department said on Thursday. Lippert underwent surgery after he was slashed in the face by a Korean nationalist in Seoul, the State Department said in a statement. "We cannot speculate on a motive at this time," it said. (Reporting by Lesley Wroughton; Editing by Emily Stephenson)