By Rajesh Kumar Singh and Sanjana Shivdas Oct 21 (Reuters) - U.S. carriers American Airlines AAL.O and Southwest Airlines LUV.N on Thursday warned mounting inflationary pressures could overshadow strong holiday demand and delay a complete return to profit. Oil prices have surged to multi-year highs this year, threatening a fragile recovery in the airline industry. Rival Delta Air Lines Inc DAL.N, which expects fuel prices to result in a pre-tax loss in the current quarter, has suggested that it might pass along the increased costs to consumers.