Ascension, a major U.S. health care system with 140 hospitals in 19 states, announced late Thursday that a cyberattack has caused disruptions at some of its hospitals.

"Systems that are currently unavailable include our electronic health records system, MyChart (which enables patients to view their medical records and communicate with their providers), some phone systems, and various systems utilized to order certain tests, procedures and medications," Ascension said in the statement.

In some cases, emergency care is affected as well, with ambulances being diverted from certain medical centers, the St. Louis-based chain said.

"Several hospitals are currently on diversion for emergency medical services in order to ensure emergency cases are triaged immediately," Ascension said. " If you are experiencing a medical emergency, please contact 911 and your local emergency services will bring you to the nearest hospital emergency room."

Some non-emergency procedures, tests and appointments are also being postponed as Ascension deals with the cyber hack, which first came to light Wednesday as "Ascension detected unusual activity in our network systems," according to a statement.

"Our investigation and restoration work will take time to complete, and we do not have a timeline for completion," the nonprofit health are system said.

Ascension also runs more than 40 senior living facilities.

The cyberattack on Ascension is just one in a series that have hit U.S. health care organizations.

In February, Change Healthcare, a subsidiary of health care giant UnitedHealth Group, was hit by a ransomware attack that disrupted billing at pharmacies nationwide and compromised the personal data of up to a third of Americans, CNN reported.

To try to protect patients' data, UnitedHealth paid $22 million to the criminals behind the attack, CEO Andrew Witty told Congress this month.

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