US actor Allison Mack has been released from prison early after serving two years for her role in a sex-trafficking case tied to a cult-like group.

Mack pleaded guilty to racketeering and conspiracy charges in April 2019 related to her efforts to recruit women to the Nxivm sex cult.

She is best known for her role in the television series Smallville.

Mack was sentenced to three years in prison but was released early.

The Federal Bureau of Prisons confirmed her release on Monday.

Nxivm, pronounced "nexium," started as a self-help programme in 1998 and later became involved in criminal activities.

Prosecutors accused Mack of helping recruit and groom women as sexual partners for the group's leader, Keith Raniere.

Female recruits were allegedly branded with Raniere's initials and expected to have sexual relations with him.

Mack's sentence was reduced after providing evidence to help prosecutors pursue their case against Raniere, who was sentenced to 120 years in prison.

Before her sentencing, Mack expressed remorse for her actions and apologized to those she had brought into Nxivm, calling her involvement a "twisted man's nefarious and emotionally abusive schemes."