ABOARD AIR FORCE ONE (Reuters) - The United States is watching "very closely" the continued possibility of a nuclear test by North Korea and will have a robust response, U.S. national security adviser Jake Sullivan said.

       "We're also watching very closely the continuing possibility of a nuclear test, to which we would also have a robust response," he said.

       (Reporting by Trevor Hunnicutt and Nandita Bose; Editing by Chris Reese)