THE victims of a fire police suspect was deliberately lit are refusing to co-operate with investigators.

A family of three adults and a child escaped uninjured from a rear unit at the East Toowoomba complex when a fire took hold just after 3am on Tuesday.

Quick work by firefighters stopped the fire spreading to neighbouring units at the Margaret St residence, but the apartment was extensively damaged.

Darling Downs Detective Inspector Lew Strohfeldt said investigations were continuing with detectives awaiting the results of forensic and scientific tests.

Inspector Strohfeldt said the occupants of the unit had given police "limited assistance" with the investigation so far, and repeated an appeal for help from the public.

"The victims of the fire have given police very limited assistance," he said.

"That unit was severely damaged but it could have been much worse.

"It was lucky we didn't have any loss of life."

Fire at units in Margaret St Toowoomba. Tuesday, 20th Nov, 2018.
Fire at units in Margaret St Toowoomba. Tuesday, 20th Nov, 2018. Nev Madsen

The cause of the fire is yet to be determined but it is believed to have been deliberately lit.

Initial investigations had indicated the fire was started in the ground-level car park and quickly spread to the unit above.

Police are also investigating whether an item was thrown onto the front veranda of the unit.

Inspector Strohfeldt said police were confident the fire, which is being treated as arson, would be solved.

Charges were likely, he said.

Police have appealed for any witnesses to the incident or people who were in the area between 2.55am and 3.30am to come forward.

Investigators are interested in speaking with anyone who may have seen a person or persons running from the area after the fire about 3.05am.

No-one was injured in the fire but three people from the targeted unit were assessed at the scene for possible smoke inhalation.

Information can be reported anonymously to Crime Stoppers on 1800 333 000 or Policelink on 131 444.