The cyber attack was first reported from Sweden, Britain and France, US media outlets reported.
An increase in activity of the malware was noticed yesterday, security software company Avast reported, adding that it "quickly escalated into a massive spreading".
Within hours, over 75,000 attacks have been detected worldwide, the company said. Meanwhile, the MalwareTech tracker detected over 100,000 infected systems over the past 24 hours.
Security researchers with Kaspersky Lab have recorded more than 45,000 attacks in 99 countries, including the UK, Russia, Ukraine, India, China, Italy, and Egypt. In Spain, major companies including telecommunications firm Telefonica were infected.
The US Computer Emergency Readiness Team (USCRT) under the Department of Homeland Security said it has received multiple reports of WannaCry ransomware infections in many countries around the world.
The ransomware is a type of malicious software that infects a computer and restricts users' access to it until a ransom is paid to unlock it. It demands users pay USD 300 worth of cryptocurrency Bitcoin to retrieve their files, though it warns that the payment will be raised after a certain amount of time. The malware spreads through email.
According to it, ransomware spreads easily when it encounters unpatched or outdated software.
A Microsoft spokeswoman said that the company was aware of the reports and was looking into the situation.
According to The Wall Street Journal, the malware believed to be behind the attacks encrypts data on infected computers and essentially holds it for ransom.
"Known as WannaCry or Wanna Decryptor, the so-called ransomware programme homes in on vulnerabilities in Microsoft Windows systems," the daily said.
In a statement, international shipper FedEx said it has been badly hit by the cyber attack.
"This event should serve as a globalwake-upcall - the means of delivery and the delivered effect is unprecedented," Rich Barger, the director of threat research at security firm Splunk, said in a separate statement.
The Department of Homeland Security (DHS) said it is actively sharing information related to this event and stands ready to "lend technical support and assistance as needed to our partners, both in the United States and internationally".
The DHS has a cadre of cyber security professionals that can provide expertise and support to critical infrastructure entities, it said in a statement.
Whistleblower Edward Snowden blamed the NSA for not preventing the global cyber attack.
"Despite warnings, (NSA) built dangerous attack tools that could target Western software," Snowden said. "Today we see the cost."
"If @NSAGov had privately disclosed the flaw used to attack hospitals when they *found* it, not when they lost it, this may not have happened," he said.
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