Most of the affected hospitals were in England, but several facilities in Scotland also reported being hit.
The US Department of Homeland Security said late on Friday that it was aware of reports of the ransomware, was sharing information with domestic and foreign partners and was ready to lend technical support.
"This is big: around the world, doctors and nurses are scrambling to treat patients without their digital records or prescription dosages, ambulances are being rerouted, and millions of people's data is potentially exposed". Hospitals, with their often outdated IT systems and trove of confidential patient data, are a particularly tempting target.
She said six of the 48 trust affected are still having problems and it not yet know if services in Lancashire among those.
The Taoiseach has said that the global cyber attack issue is a very serious matter and is being monitored very closely. The extortion attack, which locked up computers and held users' files for ransom, is believed to be the biggest of its kind ever recorded, disrupting services in nations as diverse as the U.S., Russia, Ukraine, Spain and India. He said the situation was under control.
"It is going to spread far and wide within the internal systems of organizations - this is turning into the biggest cybersecurity incident I've ever seen", United Kingdom -based security architect Kevin Beaumont said.
"It had a countdown clock ticking down, stating that all data would be deleted unless a payment was received within that timeframe", he said. "The fact is the NHS has fallen victim to this".
"A lot of people are going to go to work on Monday and click on a link in their mail - completely oblivious that all of this is going on or have heard about it and think that it's over - and suddenly wipe out their whole company", Gazeley said from Hong Kong.
Spain, meanwhile, activated a special protocol to protect critical infrastructure in response to the "massive infection" of personal and corporate computers in ransomware attacks.
"IT managers need to be extremely aware that new variants of this ransomware attack are being launched nearly hourly, so they can't just check that their computer systems are protected, then relax, assuming everything will stay that way", he said.
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It said the attacks were carried out with a version of WannaCry ransomware that encrypted files and prompted a demand for money transfers to free up the system.
Images that were posted online of the NHS pop-up look almost identical to pop-up ransomware windows that hit Spain's Telefonica, a powerful attack that forced the large telecom to order employees to disconnect their computers from its network and to resort to an intercom system to relay messages, according to Bleeping Computer.
"We have activated our major incident plan to make sure we can maintain the safety and welfare of patients".
Patrick Ward was sent home as hospital screens displayed images of a padlock.
Still, only a small number of US-headquartered organisations were hit because the hackers appear to have begun the campaign by targeting organisations in Europe, said Vikram Thakur, research manager with security software maker Symantec. "It's stressful enough for someone going through recovery or treatment for cancer".
IT experts are rushing to fix the computer systems of around 40 NHS trusts and several GP practices in England and Scotland that were hit by a cyber attack yesterday.
Grant Gowers, 50, from Clacton-on-Sea in southern England, told CNN how the ransomware attack had directly affected him.
Krishna Chinthapalli, a doctor at Britain's National Hospital for Neurology & Neurosurgery who wrote a paper on cybersecurity for the British Medical Journal, said many British hospitals still use Windows XP software, introduced in 2001. The central bank's IT attack monitoring centre "detected mass distribution of harmful software of the first and second type", a central bank statement said according to Russian news agencies.
Mr Moran said what was different about yesterday's attack was the scale and sheer speed of the attack.
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