Image copyright Getty Images Image caption "This is not our first operation, and it will not be our last," FBI director James Comey said

Authorities rescued 168 children and arrested 281 alleged pimps in cities across the US in a week-long child sex-trafficking sting, the FBI has said.

"Our children are not for sale," FBI director James Comey said, announcing the results of what the bureau dubbed Operation Cross Country.

About 400 law enforcement agencies across more than 100 cities assisted in the haul, he said.

Some of the children had been "sold" online, FBI officials said.

"These are not faraway kids in faraway lands," Mr Comey said. "These are America's children.

"This is not our first operation, and it will not be our last."

This was the eighth such operation managed by the FBI, which has recovered nearly 3,600 children and gained convictions in 1,450 cases over 11 years, the bureau said.

"Child sex traffickers create a living nightmare for their adolescent victims," said Leslie Caldwell, assistant attorney general, department of justice criminal division.

"They use fear and force and treat children as commodities of sex to be sold again and again. This operation puts traffickers behind bars and rescues kids from their nightmare so they can start reclaiming their childhood."

FBI officials said children who had been recovered would be given therapy and medical attention.

Many of the children rescued in these latest raids had formerly been part of the child welfare system but no-one had reported them missing, the BBC's David Willis reports from Washington.

"We cannot find them if no one reports them missing," John Ryan, chief executive of the National Center for Missing and Exploited Children said, adding that in only two states were agencies required to report children missing from their care.

Officials said agencies may not report the children missing because some run away repeatedly and they may believe the children will return.