A report by a research group at Harvard University said public schools in the United States have become more segregated by race over the past 14 years despite efforts across the country to increase diversity.

Harvard's Civil Rights Project said racial statistics over a 15-year period, 1986-2000, from the largest U.S. school districts show that schools are increasingly divided by race.

Authors of the study said the trend toward what is known as "re-segregation" is a result of court rulings in recent years that struck down efforts to outlaw race-based segregation.

One of the Harvard researchers, Chongmei Lee, says integration is crucial to improve education and prepare students to live in a diverse culture.