Washington - More than 2 million U.S. teenagers have suffered a serious bout of depression in the past year, including nearly 13 percent of girls, according to a federal government survey released on Tuesday.

On average, 8.5 percent of adolescents aged 12 to 17 described having had a major depressive episode in the previous year, the Substance Abuse and Mental Health Services Administration reported.

But there were "striking differences" by sex, with 12.7 percent of girls and 4.6 percent of boys affected.

Depression is the leading cause of suicide, which in turn is the third-leading cause of death for 15- to 24-year-olds in the United States.

"Combined 2004 to 2006 data show that rates of past year major depressive episode among youths aged 12 to 17 generally increased with increasing age," the researchers wrote.

Researchers at SAMHSA and RTI International in Research Triangle Park, North Carolina, prepared the report using data from the National Survey on Drug Use and Health.

More than 67,700 youths aged 12 to 17 answered questions about mood and depression. They were also asked to rate how depression affected them using the Sheehan Disability Scale, which measures impact on family, friends, chores at home, work and school.

They defined a major depressive episode as two weeks or longer of depressed mood or loss of interest or pleasure, and at least four other symptoms such as problems with sleep, energy, concentration or self-image.

Nearly half of the teenagers who had major depression said it severely impaired their ability to function in at least one of the areas on the disability scale.

The worst cases were unable to carry out normal activities for an average of 58 days in the past year.

"Fortunately, depression responds very well to early intervention and treatment," SAMHSA Administrator Terry Cline said in a statement.

"Parents concerned about their child's mental health should seek help with the same urgency as with any other medical condition. Appropriate mental health care can help their child recover and thrive."

(Reporting by Maggie Fox; Editing by John O'Callaghan)