A student shot and critically wounded two fellow students at a Maryland high school before a campus security officer ended the attack by wounding the shooter, a law enforcement official said.

The shooter shot a male student and a female student at Great Mills High School in St Mary's County, and was then wounded by a campus security officer, county Sheriff Timothy Cameron told MSNBC. All three were in critical condition at hospitals. It was not clear whether the student shooter was shot by the security officer or wounded in another fashion.

The reason for the shooting was unclear, Cameron said.

The violence was the latest in a decades-long series of shootings at US schools and colleges, coming a little more than a month after 17 students and faculty were killed in a rampage at a Florida high school.

Great Mills High School is in St Mary's County, which is about 70 miles south of Washington.

The shooting occurred amid a re-energized national debate over school shootings in the United States following the attack on February 14 at Marjory Stoneman Douglas High School in Parkland, Florida. It was the deadliest mass shooting at a US high school.

Maryland Governor Larry Hogan pledged to provide assistance. "Our prayers are with students, school personnel and first responders," Hogan said in a tweet.