A six-year-old boy,  who is cousin of Chicago Bulls player Taj Gibson, has been stabbed to death and a seven-year-old girl left critically injured after a frenzied attack in a New York apartment building.

The children went indoors for ice cream when a man struck the pair inside a lift in the Boulevard Houses public housing development in Brooklyn.

Police are now looking for the man described as black, 6ft tall, aged between 25 and 35 and was last seen wearing a grey shirt.

It is believed the suspect stabbed Prince Joshua Avitto who later died in hospital in the torso.

He was due to celebrate his seventh birthday in two weeks time.

Gibson wrote on Instagram:  "They killed my lil superman. Only two more weeks until your seventh birthday. Tears forever. This can't be life."

Mikayla Capers was also stabbed in the torso and remains in a critical condition.

New York Police Commissioner Bill Bratton has condemned the attack, and said: "The amount of violence we saw in an unusual weekend reinforces the need for an energised police department.

"We cannot have a city where two ... children going to get an ice cream are stabbed, one killed and the other fighting for her life. That is not the City of New York that we want to police."

Scott Avitto, PJ's uncle said: "It's two children. It doesn't make any sense. We're all just struggling, we're trying to pull it together. Our family is strong, but we just want justice to be done."

The children went indoors alone to get ice cream while adults watched other children playing outside in a playground.

"This is senseless. I don't know what monster would do something like this," Regenia Trevathan, the girl's great-grandmother, told CNN affiliate WCBS.

PJ's mother, Aricka McClinton, spoke to CBS 2 as the community gathered for a painful vigil.

"We got to find this man. We all have to find this man," McClinton said. "My baby is gone. My niece is critical. We've got to find this man."