Firefighters spent most of Tuesday battling a blaze that ripped through a set of inner-Sydney industrial warehouses, confronting dangerous conditions amid fears the buildings might collapse.

Up to 100 firefighters were brought in to gain control of the fire, which broke out at the set of 24 industrial units on Tuesday and sent black smoke high into the air.

Among the units are warehouses for fashion retailer General Pants Co and several automotive repair businesses.

Emergency services were called to Botany Road at Alexandria about 2.15am on Tuesday, finding the units well alight.

By 11.30am firefighters had managed to contain the fire to just two of the industrial units. However a spokeswoman for NSW Fire and Rescue said the blaze was “deep-seated” and challenging for those on the ground.

The spokeswoman said firefighters were struggling to reach the fire due to the threat of building collapse, while pallet racking inside the warehouses was at risk of caving in.

Five people in a nearby unit block were evacuated, with witnesses reporting flames jumping 60 metres into the air from the factory roof.

Fire appliances at the scene of the fire on Tuesday. Photo: AAP

Another 12 people were evacuated from a restaurant inside the complex, according to NSW Fire and Rescue’s Nick Andronicos. There were no reported injuries.

“The roof has collapsed, as has part of the internal mezzanine level,” he said.

Firefighters had successfully established a “cut-off point” to prevent the fire reaching other buildings, he said.

During the morning peak, there were significant delays to traffic on Botany Road at Beaconsfield, with only single lanes operating in both directions. Nearby, Collins Street was closed eastbound.

Commuters have been urged to avoid the area.

Huge factory fire in #Alexandria overnight; still smoking. Anxious small business owners waiting to check the damage. Fire crews have a long day ahead of them @9NewsSyd @Ninecomau pic.twitter.com/14MA8gcbrg

— Freya Noble (@freyanoble) October 28, 2019

General Pants chief executive Sacha Laing told Inside Retail that no staff had been injured in the fire and it was business as usual for the retailer’s stores and warehouse, which is in Melbourne.

However, the company posted a message on its website, warning customers it might not be able to deliver next-day orders due to an “interruption to [its] normal service”.

“Of course, we are very grateful that there have been no injuries,” Ms Laing said.

@NSW_EPA officer observing any impacts from firewater runoff and water sampling in nearby Alexandra Canal following this morning's fire in a Beaconsfield industrial unit. #fire pic.twitter.com/RchZVYIfQn

— NSW EPA (@NSW_EPA) October 29, 2019

Inside Retail said other retailers to have offices and warehousees in the suburb included GlamCorner, Koala, Coco Republic, Gelato Messina and The Iconic. None had reported disruptions from the blaze.

Authorities will remain at the fire site for the rest Tuesday and continue to monitor the factory in coming days.

The cause of the fire remains unknown.

-with AAP