A mother has been charged with first-degree murder after allegedly beheading her 6-year-old son and her dog, telling authorities that the devil was speaking to her.

Police in Kansas City said the woman, Tasha Haefs, 35, allegedly carried out the grisly crime on Tuesday night.

KCTV reports that police responded to a call Tuesday night from a woman who was afraid someone was trying to hurt her. She told police that the devil was trying to attack her. Police asked if she suffered from any mental illnesses, but she said she did not and hung up the phone.

When police arrived at her home to conduct a welfare check they found blood leading to her front door. They also heard a woman singing inside the house. Officers began knocking on her door to gain entry and she started singing louder.

An officer looked inside the house and saw the severed head of a child.

Police then burst into the woman's home through the back door, where they found Ms Haefs with blood on her legs and feet. They then found the decapitated child's body.

The officers searched the house further and found a decapitated dog in the basement along with two knives.

Jackson County Prosecutor Jean Peters Baker said because Ms Haefs was "in such extreme mental distress, she did a series of things that were very disturbing”.

The Jackson County Prosecutor's Office told detectives that the child was the woman's biological child. She allegedly admitted to killing the boy.

Ms Baker gave a statement to the press following the woman's arrest.

"The community now knows some of the terrible details of the death of this 6-year-old child," she said. "It takes our breath away."

No other children were found in the woman's home.

Ms Baker said that "anytime it's a child, it just takes on a different depth of despair”.

"It’s difficult to imagine the grief for this boy’s family. For the child’s classmates. His friends. Neighbors. The first responders who went to this crime scene," she said.

Prosecutors have asked that Ms Haefs be held without bond.