A middle-school pupil shot dead a maths teacher and injured two other students before turning his semi-automatic handgun on himself, police in the US have said.

The shooting occurred about 15 minutes before the opening bell at Sparks Middle School, Nevada, on Monday. The killer's name wasn't released, and his motive for the shooting wasn't known.

Teacher Michael Landsberry was being hailed as a hero for trying to protect children from their classmate outside the school.

About 30 pupils witnessed the shooting as they returned to classes from a week-long Autumn break.

"A kid started getting mad and he pulled out a gun and shoots my friend, one of my friends at least," a seventh-grade told local KOLO-TV.

"And then he walked up to a teacher and says back up, the teacher started backing up and he pulled the trigger," the student told the television station.

The shooting happened on the school's campus and ended outside the school building itself, according to police.

About 700 students are enrolled at the school.

Both of the wounded students from Sparks Middle School were in critical condition and one had undergone emergency surgery, said Tom Miller, the acting Sparks police chief.

Source: Agencies