WannaCry attack could worsen as people return to work on Monday

Security experts fear a worsening cyber attack by the WannaCry ransomware hack as people return to work on Monday.

The malware automatically downloads itself to infected computers, locking up nearly all of the machines' files and demanding payment of USD 300 to USD 600 for a key to unlock them.

According to a report by the New York Times, many workers, particularly in Asia, had logged off on Friday before the malicious software, stolen from the United States government, began proliferating across computer systems around the world. So the true effect of the attack may emerge on Monday as employees return and log in.

Also, copycat variants of the malicious software behind the attacks have begun to spread, according to experts.

Britain's National Cyber Security Center said on Sunday that it had seen "no sustained new attacks" but warned that compromised computers might not have been detected yet and that the malware could further spread within networks.

The cyberattack has hit 200,000 computers in more than 150 countries, according to Rob Wainwright, the executive director of Europol, the European Union's police agency.

Among the organizations hit were FedEx in the United States, Spanish telecom giant Telefonica, French automaker Renault, universities in China, Germany's federal railway system and Russia's Interior Ministry. The most disruptive attacks infected NHS England hospitals, where surgeries had to be rescheduled and some patients were turned away from emergency rooms.