NEW YORK (Reuters) - U.S. short-term interest rates futures added to earlier gains in late U.S. trading on Thursday, as fears about Britain's exit from the European Union spurred traders to consider the Federal Reserve would hold interest rates through at least early 2018.

Federal funds futures briefly implied traders had priced in a slim 2 percent chance the U.S. central bank might lower interest rates at its July 25-26 policy meeting. This compared with a 17 percent probability of a rate hike in earlier Thursday trading, according to Reuters data.

(Reporting by Richard Leong; Editing by Shri Navaratnam)