Amy Schumer attends a special screening of Snatched in New York. (AP photo)

Actors Amy Schumer and Ashley Judd have backed a campaign for an American woman convicted of killing a man who paid to have sex with her as a child to be given clemency.

Cyntoia Brown was 16 when she was convicted of murder and has been in prison in Tennessee since 2004. She is not eligible for parole until 2059 and Democrats in Tennessee have asked the state's Republican Gov Bill Haslam to offer her clemency before he leaves office in January.

"I encourage and plead with my Tennessee Governor @BillHaslam to grant #Clemency4Cyntoia," Judd -- a strong advocate in the #MeToo movement -- tweeted this week. "Cyntoia Brown was a victim of child sex trafficking, raped and forced into paid sexual exploitation to stay alive. She deserves our empathy and your mercy."

Fellow actors Schumer and Elizabeth Banks echoed the call in almost identically worded tweets. State Rep John Ray Clemmons, who was part of a Friday press conference, also took to Twitter to call for clemency. "The specific facts & circumstances surrounding & presenting Ms. Brown's current status warrant clemency in this instance, eReform." In the United States, state governors and the US president have the right to pardon prisoners or modify their sentences.

Involving anyone under age 18 in commercial sex is considered sex trafficking under US federal law. But it is common for such minors to be charged with crimes such as prostitution, experts say.

In the United States, as many as 1,000 children are arrested each year on charges of prostitution, according to Rights4Girls. Some 1.5 million people in the United States are victims of trafficking, mostly for sexual exploitation, according to anti-trafficking groups. Most sex-trafficking victims are children, they say.

The campaign to free Brown, promoted as well by singer Rihanna, rapper Snoop Dogg and reality television star Kim Kardashian, comes amid a wave of new laws vacating convictions and expunging criminal records of child-trafficking victims. The measures are often referred to as safe-harbour laws.

Twenty states have passed laws making young trafficking victims immune from crimes such as truancy and underage drinking, according to the National Conference of State Legislatures. Globally, more than 40 million people are victims of human trafficking, according to the International Labour Organization. An estimated 4 million of them are sexually exploited.