Four people have been killed and two children – aged two and 13 – injured in a mass shooting in the US city of Philadelphia, police said.

A suspect was apprehended with an AR-style rifle, a handgun, a bulletproof vest, multiple ammunition magazines in the vest, and a scanner after the shooting that spanned several blocks in or around the Kingsessing neighbourhood, Philadelphia Police Commissioner Danielle M. Outlaw said.

A second person – who may have picked up someone's gun and returned fire – also was in custody, Outlaw said.

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Three of the four killed ranged in age from 20 to 59, police said.

The age of the fourth wasn't immediately available.

Outlaw described the conditions of the wounded children as stable.

Investigators don't immediately know what led to the shooting, police said.

About 50 spent shell casings litter the shooting scenes, Outlaw said.

Earlier, police said eight people had been injured.

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At least 339 mass shootings have happened in the US so far this year, according to the Gun Violence Archive.

The archive, like CNN, defines a mass shooting as one in which four or more people are shot, not including the shooter.

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