A New York court saw scenes of anguish when a mother tearfully confronted the nanny who stabbed to death two of her young children in a bloody frenzy nearly six years ago.

Yoselyn Ortega was in the dock on Thursday for the killings of Lucia, six, and Leo, two, with a kitchen knife in the bathroom of their Upper West Side apartment on Oct 25, 2012.

Ortega, 55, does not deny the killings, but is pleading insanity.

It was a case that terrified working parents all over the world and went on to inspire a best-selling novel .

"I need to take a good look at you," said Ms Marina Krim, 41, the children's mother, who took the stand as the prosecution's first witness and addressed Ortega.

"You are a liar," she said to the Dominican-born Ortega who sat silently in court . "You are disgusting."