NEW YORK, Dec 18 (Reuters) - The U.S. airline industry faced further woes on Tuesday after Moody's Investors Service downgraded its credit rating on two leading carriers, citing the poor industry outlook after the Sept. 11 attacks on the United States. Moody's downgraded its rating on debt issued by the world's largest carrier American Airlines Inc. and its parent company, AMR Corp.(AMR). It also cut its rating of Delta Air Lines Inc.(DAL), the third largest U.S. carrier. Both airlines were already rated as junk, with Delta now slipping to the middle of those rankings and American from the top of speculative grade debt category. Moody's previously cut Delta's credit ratings on Sept 19, shortly after the attacks caused passenger traffic to plunge. Standard & Poor's already has American on a similar ratings status. The rating firm said downgrades reflected the difficult business climate facing the airline industry as carriers struggle to staunch daily cash losses from the travel slump after September's attacks. U.S. airlines recently reported huge quarterly losses as consumers stayed away from air travel in the aftermath of the attacks. Aviation experts expect the industry to report losses topping $7 billion for 2001 as low fares and slow traffic erode airline revenues. ©2001 Reuters Limited.