Witnesses say frightened children aboard a school bus in Calgary were screaming as rescuers pulled them to safety just before the front of the vehicle burst into flames.

The bus was carrying 45 passengers when Monday afternoon's fire erupted in a strip mall parking lot in the city's north end.

The Calgary Firefighters Association tweeted this photo by Twitter user @vfrcowboy of the bus fire at Harvest Hills Boulevard at 96th Avenue N.E. (@vfrcowboy/Twitter)

Witnesses say it appeared the vehicle overheated before the blaze that extensively damaged the front end.

A 10-year-old boy complained of smoke inhalation and was assessed by paramedics before he was released to his parents.

A new bus was sent to take the rest of the students home.

The cause of the fire is under investigation.

'They got the kids out right away'

"They got the door open, they got the kids out right away and instantly the flames started," said witness David Hartwick.

"They pushed the kids farther back and then the whole front of the bus was fully engulfed."

Hartwick said the heat from the flames was intense and that ashes floated down on his truck window, even though he was parked some distance from the scene.

"It was nothing and within minutes it turned into a major fire."

Bus operator Southland Transportation praised the children, saying they did exactly what they should have done.

Jonathan Weal, a company spokesman, said evacuation drills are currently being done at schools.

"They are carried out twice a year, so these students would have been recently trained in evacuating vehicles," he said.