Fears are held for two people after a monster fire ripped through an apartment building on Thursday afternoon - with firefighters struggling to get inside the top floor units.

More than 20 firefighters battled the blaze at Kawana Street, Bass Hill, in Sydney's south west, which is believed to have started before 12pm at the back of the building before spreading to the entire block in less than 10 minutes.

Twenty elderly residents have already been evacuated but there are grave concerns for two residents who are yet to be accounted for.

Firefighters can't access the units these people reportedly live in because rubble and debris is at hip height and they can't wade through until it is cleared. 

However no injuries have yet been reported, NSW Fire and Rescue said.

More than 20 firefighters battled the blaze at Kawana Street, Bass Hill, in Sydney's south west since just before 12pm

Firefighters and police officers seen next to the apartment complex which caught fire

The building contains 14 units - of which seven have been obliterated - and it is an over 55s village, where the residents are all elderly or have disabilities, Nine News reported. 

The seven top floor units have been completely damaged by the blaze and the roof caved in from the immense heat.

The bottom floor units were only lightly damaged by the inferno.

Numerous calls to Triple Zero reported smoke coming from the upper levels of the building. 

Firefighters have blocked off both Kawana Street and neighbouring Adeline Street.  

Numerous callers to Triple Zero reported smoke coming from the upper levels of the building 

Twenty elderly residents were evacuated from the building as paramedics checked in on them (pictured)

Evacuated residents were seen watching on as firefighters battled to put out the blaze 

Mary (pictured) and her husband were on their way home when they saw black smoke billowing from the apartment complex 

A nearby resident named Mary and her husband were on their way home when they saw black smoke billowing from the apartment complex. 

'The police and fire brigade were yet to arrive and we checked on people to make sure all the residents were safe,' she said.

'It was quite a heart-stopping moment.' 

Another local said he spotted smoke coming from the street, which was 'near to where his in-laws live', so he rushed over. 

'As I got here there were just people running out, kids running out, screaming, crying, "There's a fire. There's a fire,"' the man told Nine.  

'That whole top level is gone. Somehow the fire climbed through the roof. I just saw the ceiling collapse. All of the windows were breaking.'

Inspector Craig Watkins revealed that one resident was treated by paramedics for a mild asthma attack.  

A NSW Fire and Rescue spokesman confirmed to Daily Mail Australia that the blaze has been extinguished. 

Water is aimed at a top level window and the roof as flames billowed through 

Residents and their carers were evacuated and watch on as firefighters fight to put out the flames