A teenage babysitter has been arrested for allegedly forcing a four-year-old boy to perform sex acts on her while he was in her care.

Esmeralda Marie Medellin was arrested at the child's home in the Texan city of San Antonio, after he told his mother about the incident, which allegedly took place in late March.

The 18-year-old, who has been charged with aggravated sexual assault of a child, denies the allegation.

Investigators decided to charge her after she underwent a sexual assault examination at a local hospital.

The police affidavit states that the boy’s DNA was found on the 18-year-old’s breasts and male DNA was also found on the teenager’s genitals.

However, that sample was not substantial enough to be matched to the boy.