One person seriously burnt in unit blaze

One person has been seriously burnt in a fire in a unit block in Sydney's northwest.

The injured person is one of two people rescued from the first floor blaze in the Castle Hill complex while another 50 people have been evacuated as a precaution, Fire and Rescue NSW says on its Twitter feed.

Firefighters had now extinguished the fire and were working to clear smoke, it said.

FRNSW Superintendent Ian Krimmer said the burns victim had been airlifted to hospital while the other injured person was being treated at the scene.

He said it wasn't yet known what caused the fire on Hume Avenue where 24 firefighters using six fire trucks were now clearing smoke.

The evacuated residents had not yet been allowed back into the building, he said.