STOCKHOLM -- Five people were killed on Monday when a schoolbus carrying primary school students collided with a truck transporting timber in Sundsvall in central Sweden, police said.

"There were 45 people involved in the accident, of which five have been confirmed dead," Sundsvall police spokesman, Hans Sporrong told AFP.

It was not immediately known how many of the five were children.

Eighteen people were reported to be in serious condition while 22 were slightly injured, Sporrong said, adding that the cause of the crash had not yet been established.

According to Swedish news agency TT, the timber truck may have blown out a tire just prior to the accident, which occurred on a county road in Sundsvall, 350 kilometers (220 miles) north of Stockholm.

Ambulances, helicopters and paramedics were immediately dispatched to the scene.