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An update to this story can be found here.

LONDON (CNN) – Hospitals in the UK were crippled by a “large-scale” cyber attack on Friday that forced operations to be canceled and ambulances to be diverted.

Health workers reported being locked out of their systems and seeing messages demanding ransom payment to regain access. NHS England described the incident as a “ransomware” attack.

At least 16 organizations connected to the National Health Service (NHS) in England reported being affected. “The investigation is at an early stage but we believe the malware variant is Wanna Decryptor,” NHS Digital said in a statement.

“At this stage we do not have any evidence that patient data has been accessed. We will continue to work with affected organisations to confirm this.”

NHS Digital said it was working with the government’s National Cyber Security Centre, the Department of Health and NHS England to help the organizations affected “to manage the incident swiftly and decisively.” It said the attack was not specifically targeted at the NHS.

Barts Health NHS Trust in London said on its website it was “experiencing a major IT disruption and there are delays at all of our hospitals.”

It has had to cancel routine appointments and ambulances are being diverted to neighboring hospitals, Barts said.

The problem is also affecting the switchboard at Newham hospital, Barts said.

The East and North Hertfordshire Trust also said it was “experiencing significant problems with our telephone network,” in an online statement.

Another NHS Trust in Derbyshire tweeted: “We are aware of a major IT secure system attack. All IT systems have been temporarily shut down. More information will be available shortly.”