US airlines fall, FAA issues nationwide ground stop

** Shares of major US carriers fall in extended trading

** Federal Aviation Administration grounds all departing flights nationwide after a communications failure hit air traffic control centers across the US, disrupting thousands of flights

** United Airlines UAL.O, Delta Air Lines DAL.N, American Airlines Group AAL.O and Southwest Airlines LUV.N down between 1% to 1.6%

** UAL says its already airborne flights will continue, new domestic flights will not depart with FAA ground stop in place

** JetBlue Airways JBLU.O down about 3%, Frontier ULCC.O down 1.2%

** About 7,000 flights delayed or cancelled due to the nationwide ground stop, as per flight tracking service FlightAware