** Shares of U.S. airlines fall in early trading after Britain's Heathrow Airport was shut, sparking global travel turmoil

** Heathrow Airport closes after huge fire knocked out power, stranding passengers around the world and forcing flight cancellations and diversions

** Delta Air Lines DAL.N falls 2.5%, United Airlines UAL.O down 2.4%, American Airlines AAL.O loses 2.1%

** Southwest Airlines LUV.N and Jetblue Airways JBLU.O fall 1% and 2.9%, respectively

** Frontier Group Holdings ULCC.O loses 2.5%; separately, Citigroup resumes coverage of stock with a "neutral" rating

** Europe-listed airline shares also fall

** "The disruption caused by the Heathrow closure for the entire day on Friday is huge, but it will be temporary, and we expect airline stocks to stabilize after an initial knee jerk sell off" - Kathleen Brooks, research director at XTB

** Year-to-date through Thursday's close, NYSE Arca Airline .XAL index has lost 12.7% on worries about slowing travel demand due to economic uncertainty

(Reporting by Lisa Pauline Mattackal)

((LisaPauline.Mattackal@thomsonreuters.com))