An American student stabbed or slashed 20 teenagers and an adult in a bloody 30-minute rampage in the classrooms and hallways of a Pittsburgh high school on Wednesday, officials said.

Several had life-threatening injuries, Chris Kaufmann, the trauma medical director at Forbes Regional Hospital, told CNN.

The attack at Franklin Regional High School in Murrysville, a suburb of Pittsburgh, began 7:13 am (12:13 GMT) as students began arriving for what should have been a routine day.

Stevens identified the suspect as a male student.

His age and identity were not immediately released.

AFP