More Americans fell behind on their car payments in the first three months of the year, reflecting strong growth in auto loans versus a year ago and increased lending to borrowers with less-than-perfect credit.

Even so, the auto loan delinquency rate remains below the average for the January-March period going back to 2008, the credit reporting agency TransUnion said Monday.

The rate of U.S. auto-loan payments late by 60 days or more rose to 1% in the first quarter. That's up from 0.95% in the same period last year, but down from 1.14% in the last three months of 2013.

On average, the delinquency rate on auto loans in the first quarter was 1.1% between 2008 and this year, the firm said.

All told, TransUnion recorded 70 million auto loan accounts as of the first quarter, an increase of 22% from a year earlier.

As more Americans have been taking on loans to buy cars and trucks, they've also been carrying higher loan balances.

Auto loan debt per borrower grew 4.1% to $16,862 in the January-March period from a year earlier. It's been increasing steadily for three years, driven by a strong market for new and used vehicles.

New auto loans tend to have higher balances early on, which helps drive up auto loan debt.

"The continued increase in auto loan debt is a healthy sign that auto sales and the auto loan market continue to perform well," said Pete Turek, vice president of automotive in TransUnion's financial services business unit.

Stable fuel prices, low interest rates and the increased availability of credit helped propel U.S. car-buying in recent years.

U.S. consumers bought 15.6 million new cars and trucks last year. The industry entered 2014 with expectations of selling more than 16 million cars for the first time since 2007. But sales dropped 3% in January and were flat in February. March started slowly, but finished with a flourish. Sales rebounded in April, however, hitting an annual rate of 16 million.

As more drivers have gone car shopping, lenders have responded, making loans available to more borrowers, even those with less-than-perfect credit.

The number of new auto loans increased to 5.7 million in the last three months of 2013 from a year earlier. The data lag by a quarter, so the latest TransUnion figures cover the October-December period.

Nearly 32% of new auto loans issued in the fourth quarter were made to nonprime borrowers, up from 31.6% a year earlier. That's still less than the pre-recession share of nearly 37% in the July September period of 2007, but up from a low of 25.6% in the third quarter of 2009.

Non-prime borrowers are defined as those with a score lower than 700 on the VantageScore credit scale, which runs between 501 and 990, with borrowers scoring at 900 or above being considered prime borrowers, or the safest credit bet.