NEW YORK, April 25 (Reuters) - AK Steel Holding Corp. AKS.N on Friday said its first-quarter loss widened due to lower production volumes and higher costs for natural gas, pension benefits and health care.

The company, which makes flat-rolled carbon, stainless and electrical steel products, said its net loss was $40.8 million, or 38 cents per share, compared with a loss of $25.6 million, or 24 cents, a year earlier.

Sales rose to $1.0 billion from $967.7 million last year, but shipments of steel fell about 5 percent.

Middletown, Ohio-based AK Steel said it reduced shipments to the appliance, construction and manufacturing markets due to weak demand, and to the spot market as a result of lower than expected prices.

Last week, AK Steel's attempt to acquire bankrupt National Steel Corp. NSTLB.OB for $925 million was trumped by a lower bid by United States Steel Corp. X.N , in part because U.S. Steel had managed to secure a critical labor deal.

AK Steel had not been able to reach an accord with the United Steelworkers of America union, which threatened a strike at National Steel plants if the bankrupt company accepted AK Steel's bid for its assets.

The poor environment for steel companies in the United States -- including a surge in healthcare and pension costs -- has pushed more than 30 domestic steelmakers into bankruptcy since 1997, leading to a wave of consolidation within the industry.

Shares of AK Steel closed at $3.89 on the New York Stock Exchange on Thursday.