Four children were killed early Monday in a fire on Chicago's South Side and their mother and her boyfriend were critically injured, Chicago Fire Department spokesman Larry Langford said.

The children included a 5-year-old and a 15- or 16-year-old girl and two boys ages 10 and 14, Langford said. The blaze started about 3:30 a.m. in an 18-unit apartment building in the city's Roseland neighborhood. Two of the children were found in a bedroom and two were in a closet, he said. An older child was found on top of a younger child, Langford said.

"It appears one was trying to protect the other," Langford said.

About 50 other residents escaped the fire, authorities said.

The blaze appeared to start in a second floor apartment bedroom, burning into a hallway and up a stairwell to the family's third floor apartment, Langford said. The building had working smoke detectors in common areas but Langford said there was no evidence of working smoke detectors in the affected units.

The children's mother and her boyfriend were taken in critical condition to a hospital in Oak Lawn, authorities said.

Albert Miller lives on the building's second floor and said he saw two people jump from the building.

"As I was coming out the front door, two people came flying out the window. A lady and a man hit the ground," Miller told WLS-TV, which reported that the fire was extinguished by 4:30 a.m.

None of the victims' names have been released.