Sydney school kids escape from burning bus

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

A Twitter image of smoke billowing from a school bus on the M2 motorway in northwest Sydney. (AAP/ Jeremy Kwok)

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.