Up to 22 people stabbed at Pennsylvania high school

An attacker armed with two knives has gone on a rampage at a high school in the US state of Pennsylvania, stabbing 21 students and a security guard.

A suspect, named as Alex Hribal, 16, was arrested at Franklin Regional High School in Murrysville near Pittsburgh, police said.

He was later charged with four counts of attempted homicide and 21 counts of assault. The wounded, some with serious stab wounds, were as young as 14.

All are expected to survive.

"There are a number of heroes in this day. Many of them are students," Pennsylvania Governor Tom Corbett said in an evening press conference in Murrysville. "Students who stayed with their friends and didn't leave their friends."

'Frantic'

Twenty people were transported to nearby hospitals after the attack, including several who needed surgery for "significant" stab wounds, with damage to major organs, according to a trauma director at Forbes Hospital.

A 17-year-old patient was also on ventilator after a knife pierced his liver.

The young man will need additional surgery "but we're very hopeful that he will make it through this," Dr Lou Alarcon, a medical director at the University of Pittsburgh Medical Center, told a local broadcaster.