A 16-year-old shooter has opened fire at an elementary school in the United States, killing four people including three children before being killed by an armed person who had pursued the assailant into the school, police say.

The gunman entered the school  in the city of Bend in the state of Oregon on Thursday, by shooting through locked doors, police said, without offering any details beyond the shooter's age.

The dead were children aged 5, 8 and 11 and one teacher.

In addition, a 7-year-old student and a 54-year-old teacher were taken to hospital with critical injuries, Bend Police Chief Mike Krantz told a news conference.

Oregon Governor Tina Kotek, appearing at the same briefing, thanked the community member who "heroically intervened to stop further loss of life."

"No parent should have to have this kind of day, and no community should have to live through this kind of day, and as we learn more about the children and the adults who have been killed or injured, and their families and their loved ones, we are going to do a lot of crying, and we are going to cry together, and we're probably going to cry privately," Kotek said.

Emergency services were called to Silver Rail Elementary School, in rural Oregon, early on Thursday afternoon. Photo: Reuters

Krantz said the community member, whom he did not identify, had seen the suspect enter the school and pursued him.

The shooting took place in Bend, a city of about 100,000 residents in rural central Oregon, 280km southeast of Portland, in the Oregon high desert adjacent to the Deschutes National Forest.

Silver Rail Elementary School serves kindergarten through fifth grade, roughly ages 5 to 11, and has about 450 students and 40 staff members, officials said.

Police said they received a call about 1pm (local time) that "indicated that an unknown man had shot into a car" at Silver Rail Elementary School.

Moments later, emergency dispatchers received multiple calls "reporting that a person had entered the school and begun shooting," police said in a statement.

Officers arrived within three minutes and found that the suspect had been shot dead.

US Representative Maxine Dexter, a physician who represents a portion of Portland east of the Willamette River, called the shooting a "devastating day for Oregon."

"Schools should be places of learning, friendship and joy. It is incumbent on me, and every lawmaker, to do everything we can to ensure our kids are safe," she said in a statement.