WASHINGTON, United States -Two people were killed on Monday and several were injured by a gunman who opened fire at a high school in the midwestern US city of St. Louis, police said.

The gunman was shot dead by police officers who rushed to the Central Visual & Performing Arts High School, St. Louis police chief Mike Sack told reporters.

Sack said the gunman appeared to be about 20 years old and has not been identified yet.

He said an adult woman and a teenage girl were killed by the gunman, who was armed with what he called a "long gun," and another half-dozen people suffered "shrapnel and gunshot" wounds.

Sack said police arrived at the school "within a couple of minutes" of receiving a report of an "active shooter."

"Officers began to clear the building looking for the shooter," the police chief said. "Upon hearing gunfire, they ran to that gunfire, located the shooter, and engaged that shooter in an exchange of gunfire."

He said the gunman was shot and killed.

Students told the St. Louis Post-Dispatch that they locked classroom doors when they heard gunshots, while others fled the building.

Nylah Jones, a student at the school, told the newspaper the gunman fired from a hallway into the room where she was in math class but was unable to get into the classroom.

"We just heard gunshots," she said.

School shootings are frequent in the United States.

In May, a teenage gunman killed 19 children and two teachers at an elementary school in Uvalde, Texas.

A 16-year-old boy pleaded guilty in Michigan on Monday to shooting dead four classmates at his high school last year with a gun purchased as a gift by his father.