(Reuters) - The birth rate among teenagers in the United States has fallen to a historic low, with births by black and Hispanic teens down by nearly half over the past decade, the Centers for Disease Control and Prevention (CDC) said on Thursday.

Birth rates for all American teenagers are down 40 percent since 2006, thanks in part to prevention programs that address socioeconomic conditions such as unemployment and lower education levels, the CDC said.

But officials said the U.S. teen birth rate was still too high, especially for minorities, and more work was needed.

The annual survey shows the continuation of a downward trend that began in 2006 and continued through 2014, the latest year of complete data, when nearly 250,000 babies were born to girls and women aged 15 to 19.

The birth rate of 24.2 per 1,000 women in this age group is down 9 percent from 2013 and the lowest among 15- to 19-year-olds since 1940, the CDC said.

For black teens the 2014 rate was 34.9 per 1,000 and for Hispanics it was 38.0.

"While reasons for the declines are not clear, teens seem to be less sexually active, and more of those who are sexually active seem to be using birth control than in previous years," the CDC said.

Still, officials said the rates were too high.

According a report published last year by the Journal of Adolescent Health, the United States had the highest teen pregnancy rate among 21 counties with complete statistics, with 57 pregnancies per 1,000 females from 2008 to 2011. Switzerland had the lowest rate at 8 per 1,000.

In some American states, birth rates for black and Hispanic teens were more than three times higher than the rate for whites, the CDC said.

"The United States has made remarkable progress in reducing both teen pregnancy and racial and ethnic differences, but the reality is, too many American teens are still having babies," CDC Director Tom Frieden said in a statement.

With teenage birth rates varying widely in different regions, state and local officials have used data specific to their areas with greater efficiency, the CDC said.

"These data underscore that the solution to our nation's teen pregnancy problem is not going to be a one-size-fits-all. Teen birth rates vary greatly across state lines and even within states," Lisa Romero, lead author of the CDC analysis, said in the statement.

(Reporting by Daniel Trotta; Editing by David Gregorio)