Alberta Health Services says three children are in critical condition after a school bus and a truck collided north of Calgary Friday morning.

A school bus was travelling along its usual route near Crossfield, Alta., about 40 kilometres north of Calgary, when it collided with the truck shortly after 8 a.m.

A 5-year-old boy was immediately airlifted to the Calgary Children’s Hospital with potential life-threatening injuries, while six children were transported to hospital by ground ambulance.

In an update Friday afternoon, AHS upgraded the condition of two more children, and said the remaining six children are in serious but stable condition.

The bus driver was also seriously hurt and is being treated in another hospital.

Four other students who were on the bus were not injured, said a Rocky View Schools official.

A collision analyst with the RCMP is at the scene to determine what happened, Const. Robert Frizzell told CTV News Channel in a telephone interview. The intersection is currently closed to traffic.

"We're not quite sure what the contributing factor to this collision was, whether that's limited visibility or road conditions or what's actually there," he told the Canadian Press.

Adam Loria, from Calgary EMS, said motorists stopped to offer assistance.

"There was some sort of medically trained person with the military and a number of good Samaritans and eyewitnesses provided some aid," Loria said. "For everybody involved today, it was very scary, especially when you see a school bus involved and children hurt."

Early reports suggested the school bus was travelling west while the truck, identified as an F350 work truck, was headed south when they collided. The truck driver suffered minor injuries and was not taken to hospital.

Reports suggest that the children on the bus ranged in age from 5 to 13, and attend Crossfield Elementary School, said Crossfield Mayor Nathan Anderson said.

“I am very concerned. Our prayers go out to those kids and families,” Anderson said. “We just hope, God willing, they are all OK and will pull out of this without any lasting effects.”

Crisis counsellors were sent to the two schools in Crossfield immediately after the accident.

"My heart goes out to the students and parents directly impacted by this incident. We are very concerned about the well-being of our school communities," said Susan William, acting superintendent of the Rocky View School Division.

With files from The Canadian Press and CTV Calgary