A cyber-attack that hit organisations worldwide including the UK's National Health Service is "unprecedented", Europe's police agency says.
Cyber security Photo: 123RF
Europol has also warned a "complex international investigation" was required "to identify the culprits".
It's emerged that the tools used in the attack appear to have been stolen from the US National Security Agency.
There have been reports of infections in as many as 100 countries, including several in New Zealand, a live-tracking map of the attack showed.
European countries, including Russia, were among the worst hit.
Although the spread of the malware - known as WannaCry and variants of that name - appears to have slowed, the threat is not yet over.
Europol said its cyber-crime team, EC3, was working closely with affected countries to "mitigate the threat and assist victims".
In the UK, a total of 48 National Health trusts were hit by the cyber-attack, of which all but six are now back to normal, according to the Home Secretary Amber Rudd.
The attack left hospitals and doctors unable to access patient data, and led to the cancellation of operations and medical appointments.
Who else has been affected by the attack?
Some reports say Russia has seen more infections than any other country.
Banks, the state-owned railways and a mobile phone network were hit.
Russia's interior ministry said 1000 of its computers had been infected but the virus was swiftly dealt with and no sensitive data was compromised.
In Germany, the federal railway operator said electronic boards had been disrupted.
France's carmaker Renault was forced to stop production at a number of sites.
Meanwhile, the New Zealand government's cyber emergency response team (CERT) is looking into the attack here.
CERT said it was aware of the ransomware and was working on how to advise those affected.
It said the attack will likely take the shape of a phishing email with a malicious attachment or link in it and exploits machines running un-updated versions of Windows XP through to Windows 2008.
Once a single computer in a network is infected with WannaCry, the programme looks for other vulnerable computers on the network and infects them as well.
A spokesperson said people with locked up computers should lodge a report on the [www.cert.govt.nz team's] website.
No significant cyber security issues had been reported either by the Ministry of Health or the wider health sector.
Ministry of Health officials have been briefed by police about the ransomware attacks.
Police said they had not been made aware of any attacks in New Zealand and the briefing to the ministry was a precaution.
-BBC/RNZ