Two men have suffered critical burns after a factory building caught on fire in Western Sydney.

It’s believed the men were injured when dozens of gas cylinders exploded inside the building on Warren Road, Smithfield, late this afternoon.

One of the men has burns to more than one third of his body, including his airways, while the other man has burns to 18 percent of his body.

Sixty firefighters were called in to battle the blaze, which spread to a nearby building. Picture: 9NEWS (9NEWS)

The fire has now been extinguished. Picture: 9NEWS (9NEWS)

Four ambulance crews, including intensive care paramedics, treated the men at the scene before transporting them to Royal North Shore Hospital in a critical condition.

Around sixty firefighters were called in to battle the factory fire, which has now been extinguished.

An adjoining property was also damaged by the flames.

Fire and Rescue NSW Superintendent Norman Buckley said that the fire crews remain at the site this evening, while fire investigators will begin work to determine the cause.

Two men were badly burnt, reportedly by a gas bottle which exploded. Picture: 9NEWS (9NEWS)

It's not yet known what sparked the Smithfield fire. Picture: 9NEWS (9NEWS)

“It will take some time to determine the origin and the cause of this fire,” Fire & Rescue NSW Superintendent Norman Buckley said.

“Fire investigators will attend to investigate this fire. Operations are ongoing.”

All northbound lanes of the Cumberland Highway were closed at Percival Road due to the blaze, with a diversion via Percival Road and Woodpark Road in place.