A 15-year-old boy has died in hospital after being shot in the head while sleeping at his home in Sydney's south-west, as police continue to hunt the gunman.

Emergency services were called to a home in Glenfield about 6:00am where they found the boy with a gunshot wound to his head.

Police dogs were called in to help look for the gunman in Glenfield. ( ABC News )

He was treated at the scene and taken to Westmead Hospital where he died at 11:00am.

Police said the shooting was connected to a previous homicide and was a targeted attack.

They said the gunman kicked in the front door of the house before entering the teenager's bedroom while he was asleep and shooting him in the head.

Police said the intruder, whose face was covered, went upstairs where he was confronted by the boy's mother.

Officers said the pair exchanged words but would not detail their conversation.

The boy's siblings, aged seven and one, were in the house at the time.

Chief Inspector Mark Brett said police were looking for a man aged between 18 and 20.

Chief Inspector Mark Brett said police were looking for a man aged between 18 and 20. ( ABC News: Ursula Malone )

"At this stage the young person is extremely critical and police are seeking the assistance of the public in relation to the offender whose description is on the tall side, wearing a blue-coloured hoodie, last seen leaving the premises," he said.

"He's forced entry and he's forced entry to the victim's room.

"He left the premises on foot but we don't know if there was a vehicle. If anyone saw or anything or was on the street at the time we're asking not to approach the person but contact us or Crime Stoppers.

"The mother is extremely distressed. She's at the hospital with her son."

The victim is known to police.

A crime scene has been set up at the Moresby Avenue property while detectives conduct a canvass of the neighbourhood.