A cyberattack has caused significant, ongoing computer network problems for the University of Vermont Health Network, affecting its six hospitals in Vermont and New York, officials said Thursday.

The network is working with the FBI and the Vermont Department of Public Safety on the investigation, said spokesman Neal Goswami in a text.

“People who are in urgent need of care are getting it and most appointments are happening,” Dr. Stephen Leffler, president of the University of Vermont Medical Center in Burlington, said at a news conference late Thursday outside the hospital. “Most surgeries will happen tomorrow. We did slow some down today as were switching systems.”

Patients with appointments on Friday who have not been contacted should expect to have their appointments as scheduled, he said.

The FBI and two federal agencies warned in an alert Wednesday that they had “credible information of an increased and imminent cybercrime threat to U.S. hospitals and healthcare providers.” Independent security experts say the ransomware, called Ryuk, has already affected at least five U.S. hospitals this week and could potentially affect hundreds more.

The FBI would not say whether the attack on the UVM Health Network was by ransomware and would confirm only that it was investigating “a potential cyber attack” with state and local partners. Leffler said he had received no requests for ransom.

As soon as UVM Medical Center knew something was happening Wednesday, it shut down everything in the system, including all patient information, Leffler said. “We have no information that any patient information has been impacted, but we are still doing that investigation,” he said.

But the health network expects that the outage “will take some time to restore and we are working as quickly as possible to return to normal operations,” Goswami said.

UVM Medical Center had rescheduling some elective procedures and hoped to resume them on Friday, Goswami said. Patients may experience delays at Central Vermont Medical Center in Berlin and Champlain Valley Physicians Hospital in Plattsburgh, New York, he said. And patients of physician practices at Elizabethtown Community Hospital in Elizabethtown, New York, may experience slight delays, he said.

The Vermont Department of Health said it was working with the state Emergency Operations Center “to provide support for operations that impact patient care services, including testing and reporting of laboratory results.”

Vermonters may continue to get coronavirus testing through Health Department-led clinics, but the results reported through the UVM Medical Center will be affected, the Health Department said.

It said that its laboratory is working to fill the gaps and that the Health Department is contacting other labs for options “to ensure that testing and reporting of results can continue smoothly.”

In upstate New York, St. Lawrence Health System said it detected a cyberattack early Tuesday on three of its hospitals from a virus identified as a new variant of the ransomware Ryuk.

“The security measures implemented immediately made it possible to contain the virus and protect our patients and staff,” chief information officer Lyndon Allen said in a release.

Officials said no patient or employee data appears to have been accessed or compromised at its Canton-Potsdam, Gouverneur or Massena hospitals.

County Emergency Services Director Matthew Denner told the Adirondack Daily Enterprise that his department was instructed by St. Lawrence Health System to divert ambulances away from Canton-Potsdam Hospital around 4:30 a.m. Tuesday for five hours. They were later instructed to divert ambulances from Gouverneur Hospital.

Officials said there were no ambulance diversions Wednesday. A spokesperson for St. Lawrence said they were continuing to restore computer systems Thursday.

St. Lawrence said it immediately provided information to the FBI, Department of Homeland Security and other authorities.