"Multiple victims" have been reported in the US after a gunman opened fire in the area of a hospital in Chicago, according to police.
"Reports of shots fired in the vicinity of 26th and Michigan near Mercy Hospital," Chicago Police spokesman Anthony Guglielmi tweeted.
He urged people to avoid the area and said a heavy police response was incoming.
At least "one possible offender" had been shot and there were "multiple" other victims, he added.
He said a police officer had been shot and was in a critical condition.
It was not clear whether anyone was killed in the incident.
TV footage showed people filing out of the hospital into a parking lot with their hands in the air.
"We were going about our business and waiting for our results and then I heard shots fired," an unnamed witness told the local CBS affiliate.
"I turned around to my right and I see a man shooting someone on the ground, shot him multiple times.
"And then almost immediately an officer was already coming in an SUV and he exchanged fire at them and then reloaded and shot the person on the ground again and then he made his way into the hospital and more shots were fired."
The shooting came less than two weeks after a gunman killed 12 people in a California music bar packed with college students.
That rampage followed 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.
The incidents have added to the growing outcry over gun safety in the United States, where there have been more than 300 mass shootings this year.
