Firefighters are battling a massive factory blaze in western Sydney that could take days to extinguish.

The inferno at a facility containing chemical supplies in North St Marys started about 11pm, sending flames 15 metres into the air and causing at least one explosion.

About 50 fire trucks and 200 firefighters were involved in battling the blaze from three sides on Sunday morning, with Hazmat units, monitoring drones and bulk water carriers also taking part in the operation.

Firefighters said they were working to prevent the fire spreading to neighbouring buildings and handle any environmental risk posed by chemicals at the site.

Emergency services said there were no reports of injuries from the blaze.

Once the main fire is extinguished, the focus would turn to putting out spot fires, NSW Fire and Rescue’s Nick Andronicos told 2GB Radio.

He said firefighters would likely have “some involvement into the next couple of days”.

It is not known what caused the fire.