The annualized sales rate for April was forecast at 16.1 million vehicles, the firms said, with total sales at 1.395 million vehicles.

Edmunds.com, another research firm, said on Thursday that April auto sales would rise 9.1 percent to 1.4 million vehicles, for an annualized selling rate of 16.2 million vehicles.

Edmunds said April sales for General Motors Co (>> General Motors Company) and Ford Motor Co (>> Ford Motor Company) would each show a rise of about 5 percent while three automakers would have double-digit gains: Nissan Motor Co (>> Nissan Motor Co., Ltd.) at 18 percent, Toyota Motor Corp (>> Toyota Motor Corp) at 14 percent, and Chrysler Group, a unit of Fiat Chrysler Automobiles (>> Fiat SpA), at 12 percent.

"Auto sales are hitting their stride as the spring selling season begins, and the pace has returned to the level expected at this stage of the recovery," said Jeff Schuster, senior vice president of forecasting at LMC Automotive. "Fueling the growth further as the year progresses is a very robust level of new-model activity, with 63 new or redesigned models expected to hit showrooms, a 60 percent increase from last year."

March U.S. auto sales finished at an annualized selling rate of 16.4 million vehicles, and last April's annualized selling rate was 15.2 million vehicles.

The firms maintained forecasts of full-year 2014 U.S. auto sales at 16.1 million vehicles, up from 15.6 million vehicles in 2013.

(Reporting by Bernie Woodall; Editing by Andrew Hay and Jeffrey Benkoe)