A 10-year-old boy has died after being hit by a bus outside a school in Sydney's south west.

Police say emergency services were called to Cobbitty Road in Cobbitty, just before 3pm this afternoon.

The boy was outside Macarthur Anglican School when he was struck.

Footage from the scene showed paramedics loading a stretcher into the back of an ambulance.

Nearby, a bus could be seen parked on grass alongside a large looped road.

NSW Police said in a statement the boy was found at the scene with critical injuries.

"NSW Ambulance paramedics treated the boy at the scene before taking him to Liverpool Hospital in a critical condition; however, he died a short time later," they said.

The bus driver — a 60-year-old woman — has been taken to hospital for mandatory testing.

She is assisting police with their inquiries.

Local police and the Crash Investigation Unit are investigating the incident.

More to come.