CHICAGO — A gunman opened fire at a hospital in the US city of Chicago on Monday, wounding multiple people including a police officer who was said to be in a critical condition, authorities said.

Patients described being alarmed by the sound of gunshots outside Mercy Hospital and seeing a man apparently walking with a woman in the parking lot before blasting her three times in the chest.

"Once she fell to the ground, he stood over her and shot her three more times," bystander James Gray told reporters, adding that the attack played out "like a movie scene."

"They were walking and talking and he just turned around and started shooting. It wasn’t a heated exchange. It was just like we’re talking now," Gray said.

The local ABC affiliate reported that two people were dead, including the gunman, although there was no immediate confirmation.

Chicago police spokesman Anthony Guglielmi confirmed on Twitter only that a "possible offender" had been shot and asked for prayers for the felled officer, whom he didn’t name.

TV footage showed terrified patients and staff filing out of the hospital into a parking lot with their hands in the air after officers flooded the area.

Patient Hector Avitia told the local CBS television affiliate he was with his wife waiting for test results when he saw a gunman dressed in black fire on someone on the ground several times in the parking lot.

He said the gunman was carrying a handgun with multiple magazines.

"Then almost immediately, an officer was already coming in an SUV, and (the assailant) exchanged fire at them and then reloaded and shot the person on the ground again," another witness said.

"And then he made his way into the hospital and more shots were fired."

‘Scared as hell’

The shooting followed a series of recent high-profile gun rampages that have fueled calls for reform of firearms regulations in a country plagued by gun violence.

Less than two weeks earlier a gunman killed 12 people in a California music bar packed with college students.

That came after the worst anti-Semitic attack in modern US history, when a gunman opened fire on worshipers at a synagogue in the US city of Pittsburgh on October 27.

Bypassers alerted to the latest incident spoke of hearing between six and nine gunshots that initially sounded like construction noise.

"I am scared as hell. I have never been so scared, I hear of shootings going on every day at people’s workplaces, but not where I work at," an employee of the hospital’s family clinic told the ABC affiliate.

Mercy, founded in 1852, has locations throughout Chicago and provides outpatient treatment and acute inpatient care, boasting doctors who are leaders in their field.

Another hospital employee quoted by the Chicago Tribune said she was in her office when a notice came over a public address system telling those in the hospital to lock their doors.

"I don’t know what happened," the unnamed employee told the Tribune.

"They told us to run, so we did." — AFP