It took almost 30 firefighters about an hour to bring a suspicious blaze at Keilor Downs under control this morning.

Crews arrived to the Packard Street house about 6.30 to find the roof of the house, believed to be occupied by squatters, fully alight.

A search of the property confirmed nobody was inside, however firefighters were forced to battle the blaze from the outside as the second storey floor began to collapse.

An aerial appliance was used and the fire brought under control by 7.24am.

Crews will remain on the scene until the fire is completely extinguished before handing the investigation over to the police.