Major U.S. automakers' sales figures for March came in below market expectations and gave early evidence that America's long, robust boom cycle for car sales may finally be losing steam.

General Motors Co and Fiat Chrysler Automobiles shares both fell almost 4 percent, while Ford Motor Co was off 3 percent.

Auto industry publication WardsAuto put the seasonally-adjusted annualized rate (SAAR) for light vehicle sales in March at 16.53 million units. Industry consultant Autodata put industry SAAR at 16.62 million units for March.

That was below the 17.3 million analysts polled by Reuters had expected, and the first time since August that the SAAR - a crucial industry metric - had fallen below 17 million.

March "was a tough, tough, tough market," said Judy Wheeler, vice president of U.S. sales at Nissan Motor Co Ltd, which reported a 3.2 percent increase in March sales. "It's going to be an aggressive year and I think everybody realizes that."

Industry experts are increasingly concerned about rising inventory levels and consumer discounts as automakers push harder to sell their products. A pricing war in the market could undermine automakers' profits.

Last week, Moody's warned that flattening U.S. auto sales pose a significant credit risk for auto lenders.

Karl Brauer, executive publisher at auto industry consultancy Kelley Blue Book said while overall sales looked "healthy" in March discounts "have risen across the board, even on the same truck and SUV models that used to sell themselves."