Hundreds of police officers hunted on Sunday for a gunman who killed two people and wounded nine others at Brown University, plunging the eastern US campus into lockdown.

The streets around the university in Providence, Rhode Island, were filled with emergency vehicles hours after the shooter opened fire on Saturday at a building where exams were taking place.

The violence is the latest in a long line of school attacks in the United States, where attempts to restrict access to firearms face political deadlock.

Witness Katie Sun told the Brown Daily Herald student newspaper she was studying in a nearby building when she heard gunfire. She ran to her dormitory, leaving all her belongings behind.

"It was honestly quite terrifying. The shots seemed like they were coming from... where the classrooms are," she said.

Brown University student Lydell Dyer was working in the school's gym at the time, according to CNN.

"We had to go gather everybody, bring them up to the top floor, turn off the lights, and put down the blinds," he told the broadcaster, saying he hid silently in the dark with 154 others.

The gunman was still at large 10 hours after the shooting, and some 400 officers ranging from FBI agents to campus police swarmed the quaint New England campus.

Police released 10 seconds of footage of the suspect walking briskly down a deserted street, seen from behind after opening fire inside a first-floor classroom.

Brown University President Christina Paxson confirmed in a letter to community members that all 11 victims were students.

"Nine members of our community who were transported to local hospitals are all students. And we lost two students to today's devastating gun violence," Paxson said in the letter posted to the school's website.

"We learned from the hospital that six students remain in critical but stable condition. One student is in critical condition, another is considered in stable condition, and one was treated and released."

There have been more than 300 mass shootings in the United States so far this year, according to the Gun Violence Archive, which defines a mass shooting as four or more people shot.

A shelter order remains in place, the Providence Emergency Management Agency said on social media.

Providence Mayor Brett Smiley said city officials "do not feel that it is necessary" for local residents to cancel holiday-related plans over the weekend or throughout the week.

"In the hours that have eclipsed since the initial shooting, we've received no additional credible information that there is any specific ongoing threat from this individual," Smiley said.

Final exams scheduled for Sunday have been postponed, university officials said.

Emergency alert

Brown, which has a student body of about 11,000, sent an emergency alert at 4:22 pm (2122 GMT) reporting "an active shooter near Barus and Holley Engineering," which is home to the engineering and physics departments. Two exams had been scheduled at the time.

"Lock doors, silence phones and stay hidden until further notice," the Ivy League university said.

Law enforcement and first responders swarmed the scene, with local news station WPRI reporting "clothing and blood on the sidewalk."

Authorities urged anyone with information to come forward.

"We're utilizing every resource possible to find this suspect. The shelter in place is still in order and I urge people to take that very seriously. Please do not come to the area," Deputy Police Chief Timothy O'Hara told a news briefing.

He said the gunman was last seen leaving the building and that no weapon had been recovered.

US President Donald Trump has been briefed on the shooting and called the incident a "terrible thing."

"All we can do right now is pray for the victims," he said.

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.