Investigators launched a manhunt after the gunman fled the scene, the Portland Police Department said, adding that the incident was "not an active shooter situation."

The three victims - two males and one female - were "conscious and breathing" after they were transported from the scene at Rosemary Anderson High School in north Portland, it said on its Twitter feed.

Three teenagers were shot and injured outside Rosemary Anderson High School in Portland, in the northwestern US state of Oregon, police said.

The victims were struck in the torso area, Portland Fire Department spokesman Damon Simmons told AFP after the shooting, which came nearly two years to the day after the Sandy Hook school massacre in Newtown, Connecticut.

Police said the victims ran to the high school after the shooting, and that the shooter had fled the scene.

"Obviously, we're thankful we don't have any loss of life," Portland Police spokesman Pete Simpson told The Oregonian.

According to the newspaper, the school is a community-based alternative establishment that serves at-risk students sent there after failing to thrive in traditional high schools.

The police spokesman told CNN that the suspected gunman was believed to have gang links. "Preliminary information indicates that the shooter, and/or the group of people he may have been with, are gang-affiliated," he said.

But he told the broadcaster: "It's unclear whether or not that had something to do with the shooting, or if this was over a personal issue that's not gang-related."

Nearby residents and businesses scrambled for cover as the shooting erupted.

"We heard the gunshots. We dove under the desks," Tracy Mendoza, who works in a nearby business, told The Oregonian, adding that she heard five gunshots.

"They were just standing on the corner at lunchtime," she said of the victims.

The whereabouts of the gunman were not immediately clear. Simpson said police had launched a manhunt, although they were not conducting a block-to-block search, indicating the gunman was believed to be further afield.

The campus was locked down after the incident - the latest in a series of shootings at schools in the United States that regularly rekindle debate about gun control laws.

In the December 14, 2012 shooting at Sandy Hook Elementary School, gunman Adam Lanza shot dead 20 children and six adults before turning the gun on himself.

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