Police are searching for a gunman after a man was shot and killed in a southwest Sydney street while children played nearby.

Joshua George was on Gidley Crescent in Claymore, near Campbelltown, with three friends at about 1.30pm (AEDT) when he was shot, in what police believe was a targeted attack.

The 24-year-old was taken to Campbelltown Hospital, where he was later pronounced dead, a police spokesman told AAP.

Detective Chief Inspector Mark Brett says it is believed that Mr George had fought with his attacker on Saturday in nearby Airds, on the other side of Campbelltown.

"The use of a firearm is a very serious offence and the death of a person as a result of that firearm is tragic," Det Chief Insp Brett told reporters in Sydney.

"We're doing everything we can to bring this to a quick resolution."

Witnesses have described how they tried to help the dying man after he had been shot.

"We tried to assist the guy, trying to put pressure on the wound," a witness told reporters at the scene.

The victim and his attacker were known to police, Det Chief Insp Brett said.

He said police wanted to speak to a local man in relation to the shooting, as well as to Mr George's three friends who are yet to contact officers.