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.