Esthela Clark faces up to 20 years in US federal prison.

A US woman who kept a Mexican woman prisoner for years and attempted to inseminate her with her boyfriend's sperm has pleaded guilty to a charge of forced labour.

Esthela Clark, 47, paid smugglers US$3000 to sneak the woman from Guadalajara into the US so she could serve as a pregnancy surrogate.

Clark had told the victim the surrogacy would be medically supervised.

Instead, she used syringes to draw semen from condoms used by Clark's boyfriend and unsuccessfully tried to inseminate the woman over a nine-month period.

READ MORE:

* US man enslaved and sold women he called his wives

* American sex slave truck driver gets 20-year jail sentence

* Woman who enslaved disabled adults in basement gets life

* Cult leader raped followers, kept daughter prisoner for 30 years



When that failed to work, the victim was forced to have sex with two other men as the abuse got worse, according to the US Attorney's Office.

Clark isolated the Mexican woman from her family from December 2012 to January 2015, and imposed a beans-only diet on her victim, who lost about 30 kilograms from the nine-month ordeal.

The victim was forced to sleep on the dining room floor of the one-bedroom Florida apartment.

Clark even tried to get the victim's family to pay back money she paid to get her smuggled into the country from Mexico.

She was arrested in mid-2015 after a neighbour saw the Mexican woman washing cars and exhibiting physical signs of abuse. The neighbour took the woman into her home and alerted authorities.

A federal judge sent Clark to the Federal Medical Center in Texas five months after her arrest to determine if she was competent to stand trial.

Clark faces a maximum of 20 years in federal prison. A sentencing date has yet to be set.

- AP, MCT