CHICAGO, April 18 (Reuters) - Shares of major U.S. airlines, already weaker on Thursday due to reported first-quarter losses, extended declines in midday trading on reports that a small plane crashed into the Pirelli skyscraper in the financial center of Milan, Italy. "The stocks are all taking a hit because of it," said one New York airline analyst. "There's been a fear all along ... how would they hold up if there was another tragic event?" The Sept. 11 attacks using hijacked commercial airliners sent prices of major U.S. airline stocks down by half or more. The recovery since then in domestic and international travel has been slow and incomplete, causing record losses for the U.S. industry that have now totaled more than $10 billion. Shares of AMR Corp. (AMR) and UAL Corp. (UAL), the two largest U.S. airlines, which each had two planes commandeered in the Sept. 11 attacks, fell sharply along with the rest of the group. AMR, parent of American Airlines, was down 4 percent to $22.66 and United Airlines parent UAL also fell 4 percent to $14.80. ©2002 Reuters Limited.