Two gunmen have ambushed a backyard party near Pittsburgh in the United States, killing at least five people and injuring several, media and police said.

The incident happened in a residential neighbourhood in Wilkinsburg, about 13 kilometres east of the city, news station WPXI reported, adding that the suspects were still at large.

“I heard at least 20 shots,” a witness, Kayla Alexandra, told the station.

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In a statement, the local Allegheny County police said officers went to the Wilkinsburg address on Wednesday evening in response to a 911 call about a shooter.

“Once on the scene, officers found eight people suffering from gunshot wounds,” the police briefing said.

“Four of the victims, three females and one male, were dead at the scene … One female died at the hospital, the two males are in critical condition and the female is in stable condition.”

The police statement also said it appeared a party was being held at the house.

“The ballistic evidence on the scene leads police to believe that two different weapons were discharged from two different people.

“Shots were fired from an alley to the rear of [the house]. 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.

“All four of the individuals who died at the scene died on that back porch.”

Police said the investigation had just begun.

Several reporters in the area tweeted photographs of ambulances and police cars.

Dozens of shell casings littered the pavement of an alleyway at the scene, media reports said.

Wilkinsburg is a borough of about 16,000 people, with many lower- and middle-income residents, demographic data showed.