Southwest Airlines Co. warned that costs will climb faster than expected as it grapples with heavy labor expenses, undercutting gains from strong demand for leisure travel.Non-fuel unit costs will rise as much as 9% in the first quarter after an 11% jump in the final months of last year,

Southwest Airlines said Thursday it swung to a fourth-quarter profit of 42 cents a share from a year-ago loss of 42 cents a share, as overall expenses fell on lower fuel costs.

Southwest Airlines (LVU) put in a solid performance in the fourth quarter and forecast above-consensus revenue per available seat mile for the current quarter as the carrier absorbed surplus capacity across the industry with greater aircraft efficiency and higher airfares.