Four of the victims -- three women and one man -- died on the spot while another woman died in hospital, Allegheny county police said in a statement.
Two men are in critical condition and a woman is in stable condition following the incident at a home, they said.
The police said that 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, AFP reported.
The police has not been made any arrest yet.
"The ballistic evidence on the scene leads police to believe that two different weapons were discharged from two different people," the statement said.
The mass shooting has become common in the US as around 30,000 deaths per year have been recorded in the country.
Image used for representation purpose only. Photograph: Reuters
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