The suspected shooter, a 16-year-old resident of Bend, was killed by an armed bystander who confronted him inside the school, police said.

Bend Police Chief Mike Krantz said officers were first alerted to the incident after receiving a call from a person who reported being shot at multiple times by someone carrying a long gun.

Dozens of emergency calls followed, reporting that a shooter had entered the school. Officers from across the area were dispatched to the scene.

Police arrived at 1:02pm local time, within minutes of the first calls. An officer subsequently reported that a community member had pursued the suspect into the school and shot him.

Officers entered the building and confirmed that the suspect was dead.

The three children killed at the scene were aged five, eight and 11, Krantz said. A seven-year-old child and a 54-year-old teacher were taken to a local hospital with critical injuries.

The police chief said the investigation was in its early stages. Initial findings suggested that the shooter had forced entry by firing at locked doors.

Students reunited with families

Between 400 and 450 children attend Silver Rail Elementary School, which has more than 40 staff members, according to police.

Following a lockdown, the Bend-La Pine School District said officers were escorting students by bus to a separate location where they could reunite with their families.

Superintendent Steve Cook described the incident as an “unthinkable tragedy” and said the school community was heartbroken.

Parents waited anxiously at a gathering point near a church as students were brought out in buses, with younger children leaving first.

The district cancelled classes, activities and sporting events across its schools for Friday.

Silver Rail Elementary School serves children from kindergarten through fifth grade and is located about 270 kilometres south-east of Portland.

Renewed calls for gun control

The shooting prompted renewed calls for action on gun violence in the United States, where firearms are widely available and thousands of people are killed by guns each year.

The Gun Violence Archive, which defines a mass shooting as an incident in which four or more people are shot, had recorded more than 360 such incidents in the US so far this year, according to the report.

Gun safety organisation GIFFORDS expressed solidarity with the children and families affected by the shooting, saying the country could not continue treating such incidents as a normal part of life.

Oregon Congresswoman Janelle Bynum also condemned the violence, saying children continued to be killed while families were forced to live in fear.

The shooting has once again intensified calls for stricter gun laws, although previous mass shootings have resulted in limited changes to firearms legislation amid opposition from powerful pro-gun groups.