An ambush-style shooting at a backyard barbecue in the northeastern US state of Pennsylvania has killed five people and at least two suspects are still on the run, authorities said Thursday.

Two men are in critical condition and a woman is stable in the wake of the bloodshed that unfolded at a home late Wednesday in the Pittsburgh suburb of Wilkinsburg, police said in a statement.

Four of the victims -- three women and one man -- died at the scene while another woman succumbed to her wounds in hospital, they said.

Map locating Wilkinsburg in Pennsylvania, US, where at least five people were killed in a shooting ©AFP (AFP)

"The ballistic evidence on the scene leads police to believe that two different weapons were discharged from two different people," according to the statement.

No one has been taken into custody and police were urging witnesses to come forward to aid their investigation, which they said was still in its preliminary stage.

Wilkinsburg has a population of about 16,000 and is located around six miles (10 kilometers) east of Pittsburgh.

Police said shots were fired from an alley and that when partygoers tried to seek cover in the home, someone on the side or backyard adjacent to the residence fired at the back porch, where all four deaths at the scene occurred.

Lieutenant Andrew Schurman of the Allegheny County Police Department told AFP that investigators currently do not believe that anyone in the backyard or at the party fired back during the shooting.

"We're still trying to identify all the victims and what their connection is to the party," he said.

According to CBS affiliate KDKA, neighbors reported hearing 30 to 40 shots ring out.

Emergency dispatchers said seven ambulances were called to the chaotic scene, according to local broadcaster WPXI.

- 'Quiet' street -

Neighborhood resident Kayla Alexander told WPXI that the street where the shooting occurred is not known for crime.

"This street is always quiet, there's nothing but kids on this street," Alexander said.

"I'm shaken ... it's pretty sad."

Local media footage from the scene early Thursday showed a massive police presence and streets blocked off as visibly distraught groups of onlookers wept and hugged.

WPXI reported that some said they had lost family members.

"My baby! My baby!" one woman at the scene could be heard screaming according to WTAE-TV Pittsburgh.

Multiple shell casings littered the ground in the alley as detectives scoured the scene, the broadcaster reported.

The rampage represents the latest mass shooting to rock the United States, where such attacks have become commonplace.

Last month, a factory worker killed three people and wounded 17 in a shooting spree in Kansas.

Other recent mass shootings include a rampage by an Uber driver that left six people dead in Michigan and the December terror attack in San Bernardino, California, in which 14 people lost their lives.