*US carriers cancel international flights
*Delta halts 65 flights through Friday morn
*Several European countries close air space (Recasts, Updates with new figures, Delta Air, US/London, ash effects)
ATLANTA, April 15 (Reuters) - U.S. airlines canceled half their trans-Atlantic service on Thursday because of a gigantic ash cloud spewing from a volcano in Iceland that has closed airports in northern Europe, industry officials said.
A comparable number of flights, and possibly more, will likely be canceled on Friday, the Air Transport Association trade association said.
David Castelveter, an ATA spokesman, said U.S. carriers canceled about 165 flights of more than 300 operated each day. Most go to Britain.
While routes were disrupted, the main problem from the eruption under the Eyjafjallajokull glacier was the impact on airports in northern Europe. Many of them were closed or planned to do so with the cloud intensifying and continuing an eastward path.
The ash cloud forced the grounding of flights to and from Britain. London, a hub for business travel and an expanding market for U.S. airlines trying to shake a two-year downturn, is the most popular trans-Atlantic destination from New York, Chicago, Washington, Los Angeles and Boston, according to the International Air Transport Association trade group.
The impact on carrier finances was not immediately clear.
"The real question becomes how long will the volcanic ash persist," said Morningstar analyst Basili Alukos.
U.S. airlines are due to start reporting first-quarter results next week.
Earnings are expected to reflect the effects of winter storms that caused significant flight cancellations earlier this year. Airlines around the world have also experienced declines in revenue because of the recession.
Disruptions resulting from the ash cloud could last another two days, the European air safety organization said.
Ash lowers visibility but abrasive particles also damage windshields, wings, and other aircraft surfaces. It can interfere with hydraulic and electronic systems, clog ventilation and damage the moving components inside engines.
Radar cannot detect ash, creating another hazard for pilots, and damaging effects can be present up to 72 hours after an eruption.
Bill McGuire, a professor at the Aon Benfield UCL Hazard Research Center in London said if the volcano continued erupting for more than 12 months, as it did the last time, periodic disruptions to air traffic could continue. [ID:nLDE63E0QP]
Delta Air Lines Inc (DAL.N) spokesman Anthony Black said in an email that the world's biggest carrier canceled 65 flights through Friday morning from the United States to cities including Amsterdam, Brussels and even Mumbai due to the volcanic ash cloud.
United Airlines, a unit of UAL Corp UAUA.O, said it had halted 30 arrivals into Europe and 32 departures from Europe.
Continental Airlines (CAL.N) reported 32 cancellations and AMR Corp's AMR.N American said it had canceled 34 flights.
The volcano began erupting on Wednesday for the second time in a month, hurling a plume of ash up to 3.8 to 7 miles (6 to 11 kilometers). (Reporting by Karen Jacobs in Atlanta, Kyle Peterson in Chicago, Deepa Seetharaman in New York, and John Crawley in Washington; Editing by Toni Reinhold and Steve Orlofsky)