Fire crews fight restaurant fire in Sydney's north

Emergency crews are at the scene of a restaurant fire in Sydney's north.

Smoke is billowing out of the building on Alexander Street, near Burlington Road, in Crows Nest as crews work to extinguish the blaze.

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A number of businesses have been evacuated, and police have so far received no reports of injury.

Fire and Rescue NSW Superintendent Tom Cooper said crews were yet to contain the blaze.

"We've done a complete search of the building and made sure that there's nobody inside," Supt Cooper told Macquarie Radio.

"No one has been evacuated apart from workers in the restaurant - there were about five people taken out of there."

"Unfortunately the fire has got into the roof and spread through to a real estate agent next door, those workers have been evacuated as well."

Roads around the area are currently closed and a number of traffic diversions are in place.

Motorists and pedestrians are strongly urged to avoid the area this afternoon.

The cause of the fire is not yet known.