Fire at apartment block in Railway Terrace, Merrylands

The cause of a fire that forced the evacuation of dozens of residents in a western Sydney apartment block is under investigation.

A fire in an apartment building at Merrylands forced the evacuation of about 40 residents to flee their Railway Terrace homes on Wednesday about 4.50am.

Fire and Rescue NSW investigators are assisting NSW Police to determine the cause of the blaze after several triple-0 calls about a possible explosion in the basement were made.

Seven fire trucks rushed to the six-storey building where the fire destroyed two cars and damaged another eight in the basement carpark.

Heavy smoke spread throughout the property and the fire was extinguished at 7.30am.

Residents gather outside the unit block.

Police have established a crime scene as Fire Investigation and Research Unit investigators work with forensics experts to determine the cause of the fire.

Fire crews are using extraction fans to clear the smoke from the building and hazardous materials technicians used gas detectors to measure the level of toxic smoke, given off by the blaze.

Residents are securing alternative accommodation while the scene is rendered safe.