A judge in Tennessee is offering inmates at a county jail reduced sentences if they agree to undergo a free vasectomy or birth control injections, drawing fire from opponents who say the program violates fundamental constitutional rights.

The program, which offers White County Jail inmates 30 days off their time behind bars, aims to encourage participants to be successful and avoid being "burdened with children," Judge Sam Benningfield said in an interview on Thursday with Nashville's News Channel 5.

"This gives them a chance to get on their feet and make something of themselves," the judge said.

Benningfield's order took effect on May 15 but only garnered national attention after the News Channel 5 report. Dozens of inmates are said to have signed up.

Marla Neal, a spokeswoman for Benningfield's office, confirmed the details via telephone and said a statement will be issued later on Friday.

Female inmates who agree to birth control measures have also been promised shorter sentences. Photo: Shutterstock

Thirty-two women have received the birth control implant so far and 38 men were awaiting a vasectomy, according to News Channel 5.

Male inmates who volunteer are given a vasectomy while women receive the birth control implant Nexplanon under the skin of their arm, which prevents pregnancies for up to four years, the report said, citing Tennessee's Department of Health.

The state's health department did not immediately respond to a request for comment.

The American Civil Liberties Union of Tennessee denounced what it called a "so-called choice" between jail time and "coerced" contraception or sterilisation.

"Such a choice violates the fundamental constitutional right to reproductive autonomy and bodily integrity by interfering with the intimate decision of whether and when to have a child," it said in a statement on Wednesday.

District Attorney Bryant Dunaway, who oversees prosecutions in White County, told News Channel 5 he was worried about the ethical implications.

"It's concerning to me. My office doesn't support this order," Dunaway said. His office did not immediately respond to a request for comment from Reuters.

Inmates in White County, which is about 260 miles (418 kilometres) southwest of Nashville, are also given two days off their sentence if they complete an education program on the risks of raising children while using illegal drugs.

"I understand it won't be entirely successful," Bennington told the TV station in his interview. "But if you reach two or three people, maybe that's two or three kids not being born under the influence of drugs. I see it as a win-win."