Officials at the University of Vermont Health Network are investigating a “significant and ongoing system-wide network issue,” including the possibility that it caused by a malicious cyber-attack.

The outage at the network’s six hospitals in Vermont and New York has impacted on how patient care will be delivered, said a UVM spokesperson. Officials say they do not yet know when systems will be restored.

“Our staff is following well-practiced standby procedures to ensure safe patient care,” UVM Health Network said in an email. “We apologize to our patients who have been affected by this situation.

There have been some changes to patient appointments during the outage, and UVM Health Network is attempting to reach those patients who have been impacted.

UVM Health Network is rescheduling some elective procedures scheduled for Thursday, and hopes to resume normal operations by Friday.