The Rochester area’s two major health systems say they are stepping up cyber-security amid warnings that criminal hackers are targeting hospitals with crippling digital attacks.

As of mid-day Friday, University of Rochester Medical Center and Rochester Regional Health said their hospitals had not been attacked.

But at least six smaller hospitals in New York state’s North Country have been hit by the attackers, who can seize control of computer systems and freeze all activity. They typically demand payment of thousands of dollars of “ransom” before unlocking the captive computers.

If the targets don’t pay, data can be destroyed and their computer systems can be hobbled for days or weeks.

The hackers most often are able to plant ransomware, as their surreptitious software is known, by tricking employees to open phony emails doctored to look like internal communications. The ransomware is unleashed when an employee clicks a link or opens a file in the email.

© Tina MacIntyre-Yee/ Rochester Democrat and Chronicle
Eugene Polisseni Pavilion which is the main entrance at Rochester General Hospital in Rochester on March 3, 2020.

Three hospitals in St. Lawrence County were affected beginning Tuesday, according to a statement from St. Lawrence Health Systems reported by local media there.

Three other hospitals in Franklin, Clinton and Essex counties, affiliated with UVM Health Network in Vermont, were affected  beginning Wednesday or early Thursday.

U.S. cyber-security agencies warned of imminent assaults on Wednesday. The New York Times reported that the hackers are Russian criminals who have 400 U.S. hospitals on their hit list.

A health system in Oregon and another with hospitals in northern Michigan and Wisconsin also have reported ransomware attacks this week.

The on-going coronavirus pandemic gave added urgency to the alarm, as hospitals in some parts of the country are dealing with an overflow of COVID-19 patients.

Rochester Regional Health, which has two hospitals in Monroe County and…