US President Donald Trump on Friday praised the armed bystander who shot dead a teenage gunman at an Oregon elementary school as a "great hero," saying the situation could have been "much worse."

The 16-year-old attacker killed three children and a teacher Thursday at Silver Rail Elementary School in the city of Bend. Another child and teacher were critically wounded.

"As bad as it was, we had a great hero that went in and did the business," Trump told journalists, adding: "It's a terrible thing, but thank goodness for a man that had a gun and knew how to use it."

Police said the bystander pursued the gunman into the school and shot the assailant dead before officers arrived.

Trump's comments echoed an earlier statement from the National Rifle Association (NRA), a powerful lobbying group.

"Yesterday's terrible act of violence in Oregon serves as another reminder of the importance of a law-abiding armed citizenry," the NRA said in a post on X.

The bystander's actions "exemplify the heroism of everyday citizens who are prepared and ready to respond when innocent lives are on the line," it added.

The shooting has renewed debate over gun violence in the United States, where firearms are widely available and gun-related death rates are among the highest in the developed world.

"America's gun violence crisis is killing our babies. Enough is enough," gun-control group GIFFORDS -- co-founded by former congresswoman Gabby Giffords, who survived a near-fatal shooting -- said on X.

More than 360 mass shootings have been recorded across the United States this year, according to the Gun Violence Archive, which defines them as incidents in which at least four people are shot, excluding the gunman.

Polls show broad public support for stricter controls on the purchase of firearms, but the NRA is opposed to additional restrictions and lawmakers have repeatedly failed to act.

The deadliest school shooting in US history took place at Virginia Tech on April 16, 2007, when South Korean student Seung-Hui Cho killed 32 people and wounded 17 others before taking his own life.