Two gunmen reportedly ambushed a back garden party near Pittsburgh, killing at least five people and wounding several more.

Four women and one man were killed in the attack at a residential neighbourhood in Wilkinsburg, roughly 8 miles east of the US city. Two of the wounded are in a critical condition, according to NBC News affiliate WPXI.

The pair fired from an alleyway onto the party before fleeing, police said.

In a statement, the Allegheny County Police Department police department said:

The motivation behind the shooting is not yet known, officers said.

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

It is thought that the shooters - who fled the scene - are still on the run.