Unit block roof collapses as firies battle Sydney blaze

The roof of a unit block in Sydney's south-west has collapsed as firefighters battle a massive blaze.

The first call to Triple Zero came in just before midday, as a fire tore through the two-storey building on Kawana Street in Bass Hill.

There have been no reported injuries and it's believed at this stage nobody is inside the building, though some residents remain unaccounted for.

The 14-unit building serves as an over-55s village, and a number of residents are elderly or have disabilities, making evacuation more difficult.

The seven top-floor units have been destroyed, while those downstairs have been saved but may have sustained damage.

Police also evacuated a number of nearby properties.

The flames were large enough at one point that firefighters had to withdraw for safety reasons.

Traffic on the Hume Highway has been affected, and Kawana and Adeline streets are closed.