The Red Cross is assisting the three families now left homeless.
It happened around 8:30 p.m. on the 3300 block of North Water Street.
Chopper 6 was over the scene just before 9 p.m. as firefighters could be seen aggressively attacking the blaze from the second floor.
"We were scared someone was in there burning," one witness said as flames and thick black smoke billowed from the block.
Emergency responders evacuated the block to get people to safety.
Resident Jeff Abreu, speaking through a translator, told Action News what he saw.
"He was already in bed and he noticed the cops started knocking on the door. And when he saw the cops knocking on the door, that's when he saw the fire and he evacuated," the translator said.
The fire was brought under control around 9:10 p.m.
No injuries were reported.
The cause of the fire remains under investigation.