Cyber security experts said the spread of the worm dubbed WannaCry - "ransomware" that locked up more than 200,000 computers in more than 150 countries - had slowed but that the respite might only be brief amid fears it could cause new havoc on Monday when employees return to work. Spanish telecommunications company Telefonica and U.S. parcel service FedEx are among businesses hit.
In China, "hundreds of thousands" of computers at almost 30,000 institutions and organisations were infected by late Saturday, according to Qihoo 360, one of China's largest providers of antivirus software.
"We can not say and will not say that any department or agency is 100 per cent secure, because if anyone says that they don't know what they're talking about", Mr Tehan said.
Europol executive director Rob Wainwright said the situation could worsen on Monday when workers return to their offices after the weekend and log on.
Mr Tehan told ABC radio it was impossible to protect against cyber attacks with a 100 per cent success rate, but the government was reasonably confident that all required patching and security upgrades were in place.
Wainwright described the cyberattack as an "escalating threat".
"I'm anxious about how the numbers will continue to grow when people go to work and turn on their machines on Monday", he said.
Senior U.S. security officials held another meeting in the White House Situation Room on Saturday, and the Federal Bureau of Investigation and the National Security Agency were working to help mitigate damage and identify the perpetrators of the massive cyber attack, said the official, who spoke on condition of anonymity to discuss internal deliberations.
Payment is demanded within three days or the price is doubled, and if none is received within seven days, the files will be deleted, according to the screen message.
Victims were asked for payment of $300 (275 euros) in the virtual currency Bitcoin.
Given the attack's widespread nature, even such a small sum would stack up quickly, though few victims seem to be paying up so far.
Code for exploiting that bug, which is known as "Eternal Blue", was released on the internet last month by a hacking group known as the Shadow Brokers.
He warned governments against stockpiling such vulnerabilities and said instead they should report them to manufacturers - not sell, store or exploit them, lest they fall into the wrong hands.
"An equivalent scenario with conventional weapons would be the USA military having some of its Tomahawk missiles stolen", Smith wrote.
As terrifying as the unprecedented global "ransomware" attack was, cybersecurity experts say it's nothing compared to what might be coming - especially if companies and governments don't make major fixes.
It is the largest ransomware attack observed in history.
Europol said its cybercrime specialists will support affected countries as a "complex worldwide investigation" to identify the culprits begins. It has not yet detected any sign of further attacks but said it expected that some sectors would discover only at the resumption of the working week that they had been hit.
The ransomware attack struck British National Health Service organisations, along with computer networks of companies and municipalities in dozens of other countries.