Actor Allison Mack, best known for hit series "Smallville", has pleaded guilty in a case related to sex cult NXIVM.

Mack, 36, who was arrested last year, has been accused of blackmailing two women into having sex with the group's leader Keith Raniere.

Raniere was also arrested last year and has pleaded not guilty to charges against him.

Appearing at a federal court in Brooklyn on Monday, Mack pleaded guilty on two counts -- racketeering conspiracy and racketeering, according to The Hollywood Reporter.

She admitted that she coerced the women into performing services for Raniere by threatening to release damaging information about them.

"I believed Keith Raniere's intentions were to help people ... and I was wrong," a teary-eyed Mack said in the court.

Prosecutors have alleged that over the past two decades Raniere established a series of purported self-help programs under the name NXIVM, based in Albany, New York, with centres in the United States, Mexico, Canada and South America.

Mack and other cults "masters" allegedly recruited slaves by telling them they were joining a "women-only organisation that would empower them and eradicate purported weaknesses that the NXIVM curriculum taught were common in women."

The actor appeared as a series regular on "Smallville" and played the role of Chloe Sullivan on the show. She has films such as "Honey, We Shrunk Ourselves" and "The Ant Bully" to her credits.

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