The head of the European Union police agency said on Sunday the cyber assault hit 200,000 victims in at least 150 countries and that number would grow when people return to work on Monday.
Officials and experts on Sunday urged organizations and companies to update their operating systems immediately to ensure they aren't vulnerable to a second, more powerful version of the malicious software.
Microsoft had released a security patch in March, but computers and networks that hadn't updated their systems were at risk.
Clapper, who served as intelligence director under President Barack Obama, calls it a "very serious, serious problem".
The government on Sunday said it has activated a "preparedness and response mechanism" to prevent any major cyber attack from a new ransomware - "Wannacry" - which has infected computer systems around the world.
The 200,000 victims included more than 100,000 organizations, Europol spokesman Jan Op Gen Oorth told The Associated Press. The main challenge for investigators was the fast-spreading capabilities of the malware, he said, adding that so far not many people have paid the ransoms that the virus demands.
High-profile victims include hospitals in Britain, the Spanish telecoms giant Telefonica, French carmaker Renault, US package delivery company FedEx, Russia's interior ministry and the German rail operator Deutsche Bahn. Universities in China, Italy and Greece were also hit.
Britain's National Cyber Security Center and others were hailing the cybersecurity researcher, a 22-year-old identified online only as MalwareTech, who - unintentionally at first - discovered a so-called "kill switch" that halted the unprecedented outbreak.
"I will confess that I was unaware registering the domain would stop the malware until after I registered it, so initially it was accidental", wrote the researcher, who uses the Twitter name @MalwareTechBlog.
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These hackers "have caused enormous amounts of disruption- probably the biggest ransomware cyberattack in history", said Graham Cluley, a veteran of the anti-virus industry in Oxford, England.
In New Zealand, the Government Communications Security Bureau said it had not received any reports of the malware infection.
"It's quite an easy change to make, to bypass the way we stopped it", he told the AP.
Just a few days ago a global cyber attack was launched using the "WannaCry" ransomware.
In a statement, computer security group Kaspersky Labs said it was "trying to determine whether it is possible to decrypt data locked in the attack - with the aim of developing a decryption tool as soon as possible". "It should just be a case of making sure installing updates is enabled, installing the updates, and reboot". The NSA tools were stolen by hackers and dumped on the internet.
He warned that more people may be hit by the virus Monday when they return to work and switch on their computers.
Microsoft said it had taken the "highly unusual step" of releasing a patch for computers running older operating systems including Windows XP, Windows 8 and Windows Server 2003.
In Australia, Alistair MacGibbon, special advisor to Prime Minister Malcolm Turnbull on Cyber Security, said some small businesses would likely be hit "but as a whole of nation we can be confident, so far, that we have missed the worst of this".
Microsoft legal chief Brad Smith says governments should share software vulnerabilities with vendors instead of keeping them secret.
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