A fire is burning out of control in Sydney's north, with residents and a nearby aged care warned to prepare to act.

An out-of-control fire is burning near Oxford Falls in Sydney's north. Firefighters are at the scene trying to control the blaze, which is burning east towards Cromer Heights.

The Rural Fire Service has lowered the alert level for the blaze which began as a hazard reduction burn in Oxford Falls.

Residents are being urgently evacuated from Sydney's Northern Beaches as firefighters battle an out-of-control hazard reduction burn.