A girl opened fire at a school in the northwestern US state of Idaho on Thursday, injuring three people before being disarmed by a teacher, police said.

The unnamed student at Rigby Middle School near Idaho Falls was in the sixth grade, meaning she would likely be aged 11 or 12. She "retrieved a handgun from her backpack, fired multiple rounds inside of the school and out," said Jefferson County sheriff Steve Anderson.

Injuries suffered by two students and one staff member are not thought to be life-threatening, he said. "During the shooting a teacher disarmed the student, and detained her until law enforcement took her into custody," added Anderson at a press conference.

The shooting is being investigated by the FBI as well as local law enforcement. The US has suffered a spate of mass shootings in recent weeks, including at a FedEx facility in Indianapolis, an office building in California, a grocery store in Colorado and at several spas in Atlanta.

President Joe Biden last month branded US gun violence an "epidemic" and an "international embarrassment." There were more than 43,000 gun-related deaths in the United States last year, including suicides, according to the Gun Violence Archive.