A gunman at a Chicago hospital killed a police officer and two others before dying in the shooting, while to the west of the country in Denver one person was shot and killed on a street corner and three others were wounded.

Chicago Police Superintendent Eddie Johnson said the four dead in the shooting at Mercy Hospital & Medical Center included two hospital employees, a Chicago police officer, and the suspected gunman.

Johnson said the shooting began outside the hospital with a "verbal altercation" in the parking lot between people who knew each other. A friend tried to intervene and the suspected gunman pulled up his shirt and showed a weapon. Gunfire erupted and the shooter ran inside the hospital, where police confronted him.

He said one of the women killed was in a domestic relationship with the gunman.

A police officer, Samuel Jimenez, died in the attack, a police spokesman said on Twitter. The gunman also was killed, but it was unclear how.

Officials said two other people were also struck by gunfire, and an investigator at the Cook County Medical Examiner's Office later said those two also died.

Sue Jimenez, the manager of Kozy's Cyclery bike shop near the hospital, said she heard multiple gunshots separated by pauses. At one point, she heard five shots in quick succession, she said, but the shooting stopped after police arrived.

In Colorado, Denver police spokesman Doug Schepman told reporters four people were confirmed shot, with one of the victims pronounced dead at the scene. No suspects were immediately arrested.

The three surviving victims wounds were "believed to be non-life threatening", police said.

Shooting erupted about 4pm in the lower-downtown area of Denver known as Lo Do, a district of the Colorado capital filled with hotels, restaurants and high-rise residential lofts.

"We don't know right now whether there was one shooter or more than one," Schepman said.

"It's too early to say what happened and why. To have multiple gunshots fired in an area like this is quite concerning."

Schepman said witnesses had told police of a possible fifth victim, a woman, who they thought might have been shot in the leg, but he said officers had been unable to locate her.