A wave of cyberattacks on Friday affected computer systems in at least 99 countries and caused major disruptions at hospitals and medical centers in some of them, including the United Kingdom.

Russian cyber-security firm Kaspersky Lab estimated that more than 45,000 cyberattacks had been perpetrated using a ransomware hacking tool.

"So far, we have recorded more than 45,000 attacks of the #WannaCry ransomware in 74 countries around the world. Number still growing fast," the director of Kaspersky Lab's Global Research and Analysis Team (GReAT), Romania's Costin Raiu, wrote on Twitter.

The WannaCry tool exploits a vulnerability in Microsoft systems that is widely believed to have been detected and developed by the United States' National Security Agency. That exploit, codenamed "Eternal Blue," had been made available on the Internet by a hacking group known as Shadow Brokers in April.

Microsoft patched the vulnerabilities in March, but many organizations apparently had not yet installed the patch, according to Kaspersky Lab's GReAT.

 "There is nothing comparable to date. This is a massive global ransomware operation, the largest and most effective to date. Unfortunately, not all organizations patched against ETERNALBLUE/shadowbrokers exploits," that research team's principal security researcher, Kurt Baumgartner, said.

As a result of the ransomware campaign, the computers of employees of England's National Health Service were infected and locked and their data was encrypted. A message then appeared on the screen demanding $300 in Bitcoin as payment for the release of the files, numerous users reported.

Ambulances and patients in need of emergency care also had to be re-routed to other facilities due to the cyberattack.

The information technology equipment of several Spanish companies was targeted in a similar attack.

Raiu said the message that appeared on computer screens in his homeland was written in good Romanian but not by a native speaker, adding that "same for Spanish."

The United States government said on Friday it was prepared to help countries who became victims of a the massive cyber attacks, a spokesman for the US Department of Homeland Security (DHS) told Efe.

A spokesman for the Department of Homeland Security declined to divulge whether there are any US companies or infrastructure amongst the victims of cyberattacks, but said the Washington government is in talks with its international partners and willing to assist anyone who asks.

 "We are aware of the information about ransomware that affects multiple entities in Europe and Asia and we are coordinating with our international partners. The Department of Homeland Security is ready to support any request for assistance from an international or national partner, said the spokesman, adding that " DHS has a cadre of cyber security professionals that can provide expertise and support to critical infrastructure entities."

The source declined to confirm whether any country has requested assistance and stressed that any such request is confidential.

"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," the DHS added in a statement.

The method of infection and propagation of the virus occurs by taking advantage of a vulnerability of old versions of the Windows operating system.