The father of a 4-year-old Bronx girl who died after she was found unconscious in her apartment Friday “beat the baby up,” the family’s neighbor alleged to The Post Saturday.

Police officers responded to the girl’s apartment on Barnes Avenue in Williamsbridge around 9:15 a.m., after her apparently lifeless body was discovered by family members, police said.

Kyle Mazza/UNF News/Shutterstock

The girl was rushed to Jacobi Medical Center but couldn’t be saved, cops said.

A neighbor claimed he heard the father beating the girl.

“Around 8 a.m. I heard him beating the baby and I heard the baby crying,” the neighbor, who asked to remain anonymous, said.

“I had heard them before but not like that. I went to knock on the door. When I left the baby was still alive.”

Kyle Mazza/UNF News/Shutterstock

“He’s the problem,” the neighbor said.

The girl’s mom, who was wearing pajamas, and dad, who was in a bathrobe, declined to comment when reached at their apartment Saturday afternoon.

“I dont even know what happened,” the mother told The Post.

“Let me grieve,” said the mother, who appeared to have bruises on her arm. “I just gave a birthday party for my daughter at Chuck E. Cheese. Now I don’t have my daughter anymore.”

Kyle Mazza/UNF News/Shutterstock

She then went upstairs to the roof.

The father kept his head lowered and didn’t comment.

The child wasn’t publicly identified. An autopsy is scheduled.

The case is under investigation, a police spokesman said.