At least one person was taken to Vista East Medical Center, and authorities have a Pace bus on the scene as a warming shelter for persons and pets displaced in the fire.
The Red Cross is at the scene and said it would open a shelter for displaced residents if needed.
Witnesses reported seeing smoke at the 46-unit building around 7:15 a.m. The fire appears to have started in a unit on the second floor.
Waukegan Fire Marshal Steve Lenzi said crews were still fighting fires within the building's wall at 11 a.m. No cause has been determined.
The fire was struck out by 11:20 a.m. but crews will remain on the scene for a while, according to a Waukegan firefighter.