At least five people are dead and several are wounded after two gunmen ambushed a backyard party in a suburb near Pittsburgh in the US state of Pennsylvania.

Four women and one man were shot and killed, with three more people wounded in the attack, in a residential neighbourhood in Wilkinsburg, about 13km east of the city, the Allegheny County Police Department said.

Two suspects fired from an alley into a backyard party at a home and then fled, the department said in a statement on Wednesday.

"Partygoers appeared to try to run into the residence, at which point a person on the side or backyard adjacent to the residence fired at the back porch," it added.

Dozens of shell casings littered the pavement of an alleyway near the site, where police were gathering evidence and talking to witnesses, media reported.

"I heard at least 20 shots," a witness, Kayla Alexandra, told local news station WPXI.

Four people were declared dead at the scene, the police said. Four more victims were taken to nearby hospitals, but one died from her wounds, leaving two men in critical condition and a woman in stable condition, they added.