Sydney factory inferno destroys buildings

Three buildings have been destroyed in a large factory fire which engulfed several businesses in Sydney's west.

At least 100 firefighters battled the blaze which broke out at an industrial complex near Prospect Highway at Seven Hills about 12.45pm on Saturday.

Crews were able to contain the fire about 3.30pm and managed to save some surrounding buildings but three were destroyed, Fire and Rescue NSW Acting Superintendent Stephen Brown told AAP on Saturday.

The blaze was still burning with crews expected to remain on scene into the night

"It's getting smaller and is all but extinguished but it's a slow process because the roof has collapsed with the fire burning underneath," Supt Brown said.

The cause of the fire is yet to be determined and police have established a crime scene.

The plumes of black smoke which poured into the air earlier on Saturday had settled by about 6pm but smoke was lingering with residents urged to stay indoors.

Several ambulance crews were also on scene with one firefighter assessed by paramedics, a NSW Ambulance spokesman told AAP.