1036 GMT February 18, 2020
One of the victims was shot in an operating theater and later died, CNN reported.
The head of the hospital, Jiří Havrlant, told reporters outside the hospital that all of the victims were patients, and that the suspect was shooting people at close range in their heads and chests.
The incident happened just after 7 a.m., according to interior minister Jan Hamáček, who added that police and emergency response units were in attendance.
Police said they were intensively searching for a "dangerous armed perpetrator" who was carrying a "short handgun" and fled the scene in a silver-gray Renault Laguna.
Prime Minister Andrej Babiš described the incident as a "huge tragedy" and "something we are not used to here" in an interview with state broadcaster Czech TV. "We need to find out the motive, these are events that, for us, are completely from a different world."
Police said two of the dead were women and four of them were men, but declined to comment to CNN on the motive for the attack.
"The information from Ostrava university hospital [is] tragic," the country's minister of health, Adam Vojtech, tweeted Tuesday. "I am in touch with the hospital's director and I am following the situation remotely. ... thank you [to] the police and the hospital staff for their work on location."
Ostrava is an industrial and mining city in the Czech Republic's rust belt, close to the border with Poland.