A criminal investigation is underway after a six-year-old boy allegedly killed a newborn in a maternity ward.

The boy was allegedly left to “roam” around the ward and treated the baby girl “like a doll” before tragically dropping her on the floor of the Jeanne-de-Flandre Children’s Hospital in Lille, France

The baby, identified as Zayneb-Cassandra, died on Tuesday after suffering a traumatic brain injury from the fall.

Know the news with the 7NEWS app: Download today

She was born prematurely earlier in July to a 23-year-old mother, The Mirror reported.

The unnamed six-year-old is the son of another mother who was in the maternity ward at the same time.

The baby girl’s family have spoken out, saying his behaviour was disruptive in the lead up to the tragedy.

Zayneb-Cassandra’s grandparents told a local newspaper, La Voix du Nord, that the boy would arrive at 7am and “spend all day running up and down the hallways”.

They said all the mothers on the ward were complaining, and that a nurse had even warned the boy’s mother that his behaviour was a problem.

“He was entering other rooms,” the grandmother said.

“He also entered Zayneb’s room for the first time. He said she looked like a doll, and my husband, who was there, took him out.”

The horrific incident unfolded as the baby girl’s mother was signing her discharge papers on Friday.

A loud bang was heard before Zayneb was found unresponsive on the floor next to the six-year-old, who was sitting on a chair.

“It seems he tried to grab her by the diaper, and she fell on her head,” the grandparents said.

She was rushed to intensive care, but was declared dead.

An investigation has been launched by the juvenile unit of the Lille Judicial Police Service.

The Jeanne-de-Flandre Children’s Hospital has also said it is undertaking an internal administrative investigation.