Twenty-one students and a security guard were stabbed early Wednesday by a 16-year-old boy student at a high school in the US state of Pennsylvania.

The attacker, armed with two knives of about 8 to 10 inches (20-25 cm), started the rampage at about 7:13 am local time (1113 GMT) at Franklin Regional High School in Murrysville 15 miles (24 km) east of Pittsburgh.

The boy, identified as Alex Hribal, was taken into custody by police and treated for a minor hand wound shortly after the incident.

The victims, mostly aged between 15 to 17, had been taken to hospital. At least seven were critically injured, including a 17-year-old boy whose liver was pierced, narrowly missing his heart and aorta, doctors said.

Authorities said Assistant Principal Sam King tackled the boy and disarmed him with the help of a security guard. He would be charged as an adult with four counts of attempted homicide and 21 counts of aggravated assault, according to the Murrysville Police Department.

The school, which has about 1,200 students, would be closed for the next two or three days as an investigation was underway.

The motive of the knife attack was unknown. Pennsylvania Governor Tom Corbett said he had ordered state police to help local law enforcement to probe the incident.

The attack has been the third mass stabbing at US schools since last year. Fourteen people were wounded at a community college in Texas last April and a 17-year-old student stabbed three others in a Texas high school in September.