NEW YORK (AFP) - A New York nanny who killed two young children went on trial on Thursday (March 1), accused of murder but pleading insanity.

Any parent's worst nightmare, the case went on to inspire a best-selling novel.

Yoselyn Ortega took a kitchen knife and stabbed to death Lucia, six, and Leo, two, in the bathroom of their Upper West Side apartment on Oct 25, 2012.

The children were found bloodied in the bath by their mother Marina Krim, who rushed home in a panic with her third child Nessie after discovering Ortega had not taken Lucia to her dance class as planned.

Krim found Ortega taking the knife to her throat in an apparent suicide attempt.

Six years later, the former nanny sat silently in court on Thursday wearing a grey pullover with a brown headband over her long brown hair.

New York prosecutor Courtney Groves told the court that Ortega "brutally butchered the children" and that there was "overwhelming evidence that she planned these murders and executed them at a time she knew she would not be interrupted."

Defence lawyer Valerie Van Leer-Greenberg said Ortega suffered from "chronic mental illness" - had heard voices since she was 16 years old and suffered from depressive disorders, psychotic thinking and hallucinations that had gone untreated since she was a teenager.

Ortega had worked for the Krim family for two years.

Although from opposing worlds - the Krims were wealthy parents who moved from California while Ortega had left behind her own child in the Dominican Republic to seek work in the United States - they had outwardly gotten along well.

Marina and her husband Kevin hired Ortega to help juggle the children's various extracurricular activities.

Ortega, whose son came to join her in the United States when he was 17 years old, lived in her own apartment in the Bronx when she first worked for the Krims, but later had to move in with her sister.

INSPIRATION FOR AWARD-WINNING NOVEL

The killings served as the inspiration for Chanson Douce - a best-selling novel that earned author Leila Slimani France's top literary prize, the Goncourt, in 2016.

The novel was translated as Lullaby in Britain and in the United States as The Perfect Nanny, where it was less commercially successful than in Europe.

Slimani was only the 12th woman to have won the Goncourt in its more than a century-long history, joining a list that includes Marguerite Duras and Simone de Beauvoir.

Her novel, which transposes the essentials of the Krim killings to a wealthy Parisian family, crackles with class tension and has been called the nightmare of working parents.