When you hear stories of children being exploited for sex the United States wouldn't be the first country that comes to mind. But that kind of prostitution does happen in America, and authorities say child sex trafficking has become a national problem.

The FBI announced Monday that in three days, it had rescued 79 teens who were being used as prostitutes, and nabbed more than 100 pimps.

The 72-hour sweep, which took place over the weekend, was known as Operation Cross Country. It was conducted in 57 cities, including parts of California, Ohio, Atlanta, and Louisiana, and involved hundreds of state and local law enforcement agents.

Child prostitution remains a major threat to children across America, said Kevin Perkins, acting executive assistant director of the FBI's Criminal, Cyber, Response, and Services Branch. It is a violent and deplorable crime, and we are working with our partners to disrupt and put behind bars individuals and members of criminal enterprises who would sexually exploit children.

Ernie Allen, president of the National Center for Missing and Exploited Children, or NCMEC, said many of the children got started in prostitution because they needed help. He said they ran away from home and needed shelter. It is in these most vulnerable stages that pimps would approach the children, offering them kindness, food and shelter, and most of all love.

The operators, the pimps, the exploiters, target them, win their confidence, Allen told NBC News. But at some point, suddenly these kids discovered that they can't walk away, that they don't have the right to walk away.

Technology Changes How Pimps Sell Children

Allen said these children are truly 21st century slaves.

They are being abused, manipulated through violence and threats of violence, he said. They don't get the money, so they really are being used as commodity for sale or trade.

And pimps don't necessarily have the children on the streets in plain view, for all to see that prostitution is happening.

Allen said technology has change how these children are being marketed. Pimps are no longer taking the risk of having children strut down city streets.

Now would-be customers can shop from the privacy of their own homes or hotel rooms, Allen said. The exploiters can post the photos of kids on advertising websites. The reason that they are engaged in this kind of activity is it's easy, it's low risk and enormously profitable.

What investigators are now trying to do is up the risk for these pimps and remove their means of profit.

More than 2,000 Children Recovered

The FBI's Criminal Investigative Division partnered with the NCMEC and the Department of Justice's Child Exploitation and Obscenity Section in 2003 to launch the Innocence Lost National Initiative.

To date, more than 2,200 children have been recovered from the nation's streets. Additionally, 1,017 pimps, madams and their associates have been convicted, according to the FBI. Offenders have been handed down some lengthy sentences as well. Some have received multiple sentences of 25 years to life. More than $3 million in assets have also been seized so far.

Among the places of prostitution targeted by agents are truck stops, casinos, street tracks, and Internet websites.

When authorities make an initial arrest, they do so for violations of the local and state laws against prostitution. However, a deeper look reveals organized prostitution activities involving women and children in many states.

In the United States it's not just happening in the big cities, Allen told NBC News. Look around you. If you see it, if you suspect it, if you know about it, report it to police and report it to us at the National Center for Missing and Exploited Children.