U.S. banks looking to get in on a booming market for financing new-car sales have run into a formidable competitor: the auto manufacturers themselves.

Financing arms of car companies, including Toyota Motor Corp., Honda Motor Co. and Ford Motor Co., made half of all new U.S. car loans in the first quarter, up from 37 percent a year earlier and the largest percentage of the market in four years, according to credit data firm Experian.

These companies also write the vast majority of leases, which contributed a record 26 percent of new car sales in the quarter, up from 23 percent last year and 20 percent in 2012.

The financing arms are providing subsidies from the manufacturers, lowering monthly payments and extending loan terms to make it easier for buyers to drive away in a shiny, new vehicle. As a result, major banks are increasingly moving into riskier parts of the market to make loans.

U.S. Bancorp, for example, for the first time ever decided to start financing used cars, an area of the market that the automakers’ finance companies have little interest in. It also started offering loans to less creditworthy borrowers.

And Wells Fargo has been leveraging off a nationwide deal with General Motors (GM) to provide loans subsidized by the No. 1 U.S. automaker. Wells sees this as a way to gain more of the used car loan business at GM dealerships.

The aggressive push by car companies is beginning to raise questions among industry analysts and consultants about whether it is sustainable.

If interest rates rise, the automakers could find the incentives too costly unless they are prepared to take a hit to profits — with any pullback in the deals being offered customers running the risk of hurting demand. And, if used car prices weaken, the financing units could be hit with losses on vehicles coming back from leases and repossessions.

The automakers’ financing companies are doing substantially more than they were just a year or two ago, said April Ancira, vice president in the San Antonio office of Ancira Motor, a Texas-based group with 11 dealerships selling GM, Nissan, Fiat, Chrysler, VW and Ford cars.

“They’re being very aggressive with incentives,” Ancira said.

Pete Carey, vice president for sales at Toyota Financial Services, said incentives are playing a bigger role as automakers look to stand out in a crowded market where the basic quality of cars is uniformly good.

(SD-Agencies)