Governments and computer experts girded Monday for a possible worsening of the global cyberattack that has hit more than 150 countries, as Microsoft warned against stockpiling vulnerabilities like the one at the heart of the crisis.
European policing and security agencies said the fallout from a ransomware attack that has already crippled more than 200,000 computers around the world could deepen as people return for another work week.
The indiscriminate attack began Friday and struck banks, hospitals and government agencies, exploiting known vulnerabilities in older Microsoft computer operating systems.
US package delivery giant FedEx, European car factories, Spanish telecoms giant Telefonica, Britain's health service and Germany's Deutsche Bahn rail network were among those hit.
Europol executive director Rob Wainwright said the situation could worsen on Monday when workers return to their offices after the weekend and log on.
"We've never seen anything like this," the head of the European Union's policing agency told Britain's ITV television, calling its reach "unprecedented".
Wainwright described the cyberattack as an "escalating threat".
"I'm worried about how the numbers will continue to grow when people go to work and turn on their machines on Monday," he said.
The warning was echoed by Britain's National Cyber Security Centre: "As a new working week begins it is likely, in the UK and elsewhere, that further cases of ransomware may come to light, possibly at a significant scale."
The attack looks like this: images appear on victims' screens demanding payment of $300 (275 euros) in the virtual currency Bitcoin, saying: "Ooops, your files have been encrypted!"
Payment is demanded within three days or the price is doubled, and if none is received within seven days the locked files will be deleted, according to the screen message.
Bitcoin, the world's most-used virtual currency, allows anonymous transactions via heavily encrypted codes.
Experts and governments alike warn against ceding to the demands and Wainwright said few victims so far had been paying up.
Security firm Digital Shadows said on Sunday that transactions totalling $32,000 had taken place through Bitcoin addresses used by the ransomware.