Sydney residents left homeless after fire tears through apartment block

Around a dozen people have been left without a home, after a fire tore through a unit block in Sydney's south early this morning.

Flames ripped through the top floor of the property in Carlton, with residents in six units evacuated.

Firefighters said it was one of the most ferocious blazes they have seen.

"I don't think in 32 years of firefighting I have seen a unit more destroyed on arrival," firefighter Glenn Moran told 9NEWS.

Structural engineers are now working to determine how safe the remainder of the building is before residents can go home, with concerns for asbestos.

It is not yet clear what caused the blaze.