Sydney school kids escape from burning bus

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School children have fled a burning bus which caught fire on the M2 motorway in northwest Sydney.

Fire & Rescue NSW said on its Twitter page about 40 people had got off the school bus and heavy traffic was banking up at Carlingford about 3.30pm (AEDT) on Friday.

Inspector Phil Brooks from the NSW Police Traffic and Highway Patrol Command said a group of school children were walking away from the burning bus.

"Looks like we've got a load of school children off the bus. They're walking back up the M2 now to get out of the way," he told Macquarie Radio.

"But this bus is well and truly on fire as we speak."

A Fire & Rescue spokeswoman told AAP the engine compartment of the bus had caught fire and the first emergency call came in at 3.13pm.

She said school children and teachers had safely disembarked from the bus, but it was not yet known what school they attended or whether they were primary or high school-aged students.

Three fire crews are battling the blaze, she said.

- AAP