Some cyber security experts and privacy advocates said the massive attack reflected a flawed approach by the USA to dedicate more cyber resources to offence rather than defence, a practice they argued makes the internet less secure.
Hospitals across Britain found themselves without access to their computers or phone systems.
The neurologist said the hackers appeared to be using ransomware, a specific type of malware which locks the owner out of their computer until they pay to get back in - but he said he was anxious the attack could be something even more sinister.
Britain's health services were attacked Friday by a massive worldwide cyber attack that encrypted information and froze computers at hospitals across the country.
"We'd like to reassure patients that if they need the NHS and it's an emergency that they should visit A&E or access emergency services in the same way as they normally would and staff will ensure they get the care they need", NHS incident director Anne Rainsberry said in a statement.
Earlier, a number of hospitals across England were forced to divert emergency patients after being hit by a suspected cyber attack. Some chemotherapy patients were even sent home because their records could not be accessed. NHS Lanarkshire and NHS Western Isles also confirmed they have been affected.
Up to 99 countries may have been affected by the ransomware cyberattack that has struck the NHS, according to some experts.
As similar widespread ransomware attacks were reported in Spain, Romania and elsewhere, experts warned that online extortion attempts by hackers are a growing menace.
He added the attack "was not specifically targeted at the NHS and is affecting organisations from across a range of sectors".
In Scotland more than half of regional health boards were affected.
Following the cyber attacks pictures were posted on social media showing screens of NHS computers with images demanding payment of Dollars 300 (275 euros) worth of the online currency Bitcoin, saying: "Ooops, your files have been encrypted!"
Alan Woodward, visiting professor of computing at the University of Surrey, said there was evidence the ransomware was developed using a Microsoft flaw that was recently exposed in a leak of information from US intelligence agencies.
Private security firms identified the ransomware as a new variant of "WannaCry" that had the ability to automatically spread across large networks by exploiting a known bug in Microsoft's Windows operating system.
A Barts spokesman said it was experiencing "major IT disruption" and delays at all four of its hospitals.
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An exterior view shows the main entrance of St Bartholomew's Hospital, in London, one of the hospitals whose computer systems were affected by a cyberattack on Friday.
It demands payment in three days or the price is doubled, and if none is received in seven days the files will be deleted.
"Despite warnings, (NSA) built unsafe attack tools that could target Western software", Snowden said.
"For so many organisations in the same day to be hit, this is unprecedented", he said.
The ransomware encrypted data on the computers, demanding payments of $300 to $600 to restore access.
Spain was also affected by the ransomware, and the country activated a special protocol that protects critical infrastructure, as a response to the massive infection of corporate and personal computers.
Worldwide shipper FedEx Corp said some of its Windows computers were also infected.
The Spanish government said their attacks were carried out with a version of WannaCry ransom ware that encrypted files and prompted demand for money transfers to free up the system.
Spain's Telefonica was among the companies hit.
The news is also likely to embolden cyber extortionists when selecting targets, Chris Camacho, chief strategy officer with cyber intelligence firm Flashpoint, said. "It's stressful enough for someone going through recovery or treatment for cancer".
A police spokesperson said they're not aware of any incidents in New Zealand related to the attacks but have informed Ministry of Health officials as a precaution.
Across the US federal government, about 90 percent of all spending on cyber programs is dedicated to offensive efforts, including penetrating the computer systems of adversaries, listening to communications and developing the means to disable or degrade infrastructure, senior intelligence officials told Reuters in March.
The National Cyber Security Centre said it was aware of a "cyber incident" and was working with NHS Digital and the National Crime Agency to investigate.
Britain's National Health Service is a source of pride for many Britons but faces substantial budget issues and has had previous problems with its huge IT system. In February 2016, the Hollywood Presbyterian Medical Center in California said it had paid a $17,000 ransom to regain control of its computers from hackers.
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