GENEVA — Car crashes are the leading cause of death worldwide for people between 10 and 24, the U.N. health agency said Thursday, adding that most such fatalities occur in developing countries with poor road safety conditions.

Nearly 400,000 young people under 25 are killed in traffic accidents every year and millions more are injured or disabled, the World Health Organization said.

“The lack of safety on our roads has become an important obstacle to health and development,” said WHO’s director-general, Dr. Margaret Chan. “Our children and young adults are among the most vulnerable.”

The agency’s 40-page “youth and road safety” report was issued as part of a new initiative by the United Nations to improve global road safety.

“Road traffic crashes are not ‘accidents,”’ Chan said. “We need to challenge the notion that they are unavoidable.”

WHO said deaths from lower respiratory infections, AIDS and diarrhea are much higher for children up to age 10, but then fatalities from road accidents jump dramatically. Young males are at higher risk than females in every age group under 25, it added.

The report said the majority of road deaths could be prevented if proper safety measures were taken. Many accidents involve children playing on the street, young pedestrians and inexperienced motorcyclists and car drivers, it said.

WHO said such measures as enhancing the safety of children traveling to school and lowering blood alcohol limits for young drivers could help cut the death toll.

It also suggested introducing graduated license programs, lowering speed limits, cracking down on drunk driving and enforcing use of protective devices such as seat belts, child restraints and motorcycle helmets.

Authorities also should improve roads and create safe areas away from traffic for children to play, the report said.

WHO estimated road accidents impose $518 billion in economic and health costs around the globe. In some poorer countries, the damage is as high as 1.5 percent of national output and exceeds the total amount of money received in international development aid, it said.