An apparent drive-by shooting Monday near a Colorado high school left six teenagers in hospital, police said, as the spectre of gun violence raised its head again in the United States.

Witnesses spoke of around 30 shots being fired, and police said they had recovered multiple rounds of different calibres at a park in Aurora, a municipality in Denver's suburbs.

Aurora Police Chief Vanessa Wilson said there appeared to have been more than one shooter, and that the department's gangs unit was involved in the investigation.

"My understanding is it was a drive-by shooting as well as there may have been people on foot," she told reporters. "We do have suspects that are at large."

Wilson said five teens aged between 14 and 17 had been taken to hospital, one of whom was in emergency surgery.

A sixth person, age 18, was treated for minor injures after arriving at the hospital on their own.

The victims, all of whom were expected to survive, are students at nearby Central High School.

"Obviously this is a very, very concerning incident for the city, as well as for this nation," she said.

"I think we're seeing a public health crisis really when we think about gun violence."

Colorado is no stranger to such violence, having suffered two of the most infamous mass shootings in US history.

In 1999, two boys shot and killed 12 classmates and a teacher at Columbine High School.

Then in 2012, a heavily armed man stormed a movie theatre in Aurora, murdering 12. The gunman is now serving a life sentence without the possibility of parole.

Last March a gunman killed 10 people, including a police officer, at a grocery store in Boulder County, 50 kilometres (30 miles) outside Denver.

Mass shootings and other gun violence are huge problems in the United States, issues that gun control advocates say are fueled by the prevalence of firearms and relatively lax laws.

Tighter gun control is overwhelmingly popular with Americans -- but efforts to overhaul rules are regularly thwarted by right-leaning politicians, who view restrictions as an unwarranted curb on personal freedoms.