Gunman kills three at Chicago hospital

An argument in a hospital parking lot escalated into a shooting that killed three people, including a police officer, in the US city of Chicago on Monday.

The violence only ended when police engaged in a shootout with the gunman inside Mercy Hospital.

"We have four deceased individuals: police officer, two female staff employees at the hospital, and the offender," police Superintendent Eddie Johnson told journalists.

The shooter killed the first victim - a woman who was in a "domestic relationship" with him - during an argument in the parking lot, then fired at police when they got to the hospital, and ran inside, Johnson said.

Additional police - including SWAT - arrived at the scene, and officers "engaged the offender for several minutes in the hospital, with gunshots being fired by the offender and by the police," he said.

During the shootout, the gunman killed a second woman when she got off an elevator, Johnson said. The shooter also fatally wounded, but it was unclear if he killed himself or was killed by police.

Mayor Rahm Emanuel identified the two female victims as a doctor and a "pharmaceutical assistant." 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 shooting 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."

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.