WASHINGTON (AFP) - Sex trafficking of children is growing in many urban areas of the United States, according to a survey report on Tuesday that underscored calls for more robust action.

Forty per cent of counties with populations higher than 250,000 reported an increase in sex trafficking of youngsters under 18 in the past two years, said the survey from the National Association of Counties.

Fifty-one per cent of like-sized counties said the problem has remained the same - while only 11 per cent of smaller counties reported an increase over the same period, and 77 per cent saw no change.

For all age groups, human sex trafficking was deemed "a major problem" by authorities in 48 per cent of all larger counties.

"Sex trafficking is a problem across America, particularly for large urban counties," Don Knabe, a member of the elected Board of Supervisors of Los Angeles County, told reporters in Washington.

In his own jurisdiction of 10 million, which includes the city of Los Angeles and its many sprawling suburbs, girls - some as young as 12 years old - have been "bought and sold on the streets ... for sex", he said.

The survey, based on interviews with sheriff's departments in 400 counties earlier this month, also indicated a link between sex trafficking and minors who have been in foster care, group homes or involved in abuse cases.

"Protecting young children and keeping them safe from their pimps is a huge challenge to us and something for which we need to find solutions and funding," Knabe said.

Ted Poe, a Republican congressman from Texas and former judge who introduced legislation in 2013 to toughen penalties for clients of prostitutes, denounced sex trafficking as "modern-day slavery". According to the National Centre for Missing and Exploited Children, at least 100,000 minors, more often than not children, are victims of sexual exploitation in the United States every year.