British Prime Minister Theresa May has said her government is not aware of any evidence that patient records have been compromised in the massive cyber attack on the National Health Service.
Mrs May said the ransomware hit was "not targeted" at the health service but was part of a wider assault on organisations across a number of countries.
The National Cyber Security Centre is working to support the NHS.
Mrs May said: "We are aware that a number of NHS organisations have reported that they have suffered from a ransomware attack.
"This is not targeted at the NHS, it's an international attack and a number of countries and organisations have been affected.
"The National Cyber Security Centre is working closely with NHS digital to ensure that they support the organisations concerned and that they protect patient safety.
"And, we are not aware of any evidence that patient data has been compromised.
"Of course it is important that we have set up the National Cyber Security Centre and they are able to work with the NHS organisations concerned and to ensure that they are supported and patient safety is protected."
Ransomware - Questions and answers
The Health Service Executive has said it has taken moves to protect its IT systems following the attack.
Hospitals and GP surgeries in England and Scotland were among at least 16 health service organisations hit by the ransomware attack, using malware called Wanna Decryptor - with reports potentially dozens more were affected.
Staff were forced to revert to pen and paper and use their own mobiles after the attack affected key systems, including telephones.
Pictures posted on social media showed screens of NHS computers with images demanding payment of $300 worth of the online currency Bitcoin, threatening to delete files within seven days.
A spokesman for NHS Digital, which manages health service cyber security, said: "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."
He added the attack "was not specifically targeted at the NHS and is affecting organisations from across a range of sectors".
The attack came as several companies in Spain were hit by ransomware attacks. Telecoms firm Telefonica was one of those reporting problems.
Wanna Decryptor is a piece of malicious software that encrypts files on a user's computer, blocking them from view and threatening to delete them unless a payment is made.
The virus is usually covertly installed on to computers by being hidden within innocent-looking emails containing links, which users are tricked into opening.