The United States officially accused North Korea of carrying out the massive Wanna Cryattack that infected some 300,000 computers in 150 countries earlier this year.
North Korea was widely suspected of being behind thecomputer virus and ransomware, which demanded payment torestore access. It has been denounced as such by Britain, but the United States had yet to follow suit.
Homeland Security Advisor Tom Bossert made the announcement in a Wall Street Journal op-ed, and was expectedto provide more details in a briefing with reporters on Monday.
"The attack was widespread and cost billions, and NorthKorea is directly responsible," he wrote.
"We do not make this allegation lightly. It is based onevidence." Among the infected computers were those at Britain'sNational Health Service (NHS), Spanish telecoms companyTelefonica and US logistics company FedEx.
"These disruptions put lives at risk," Bossert wrote.
"North Korea has acted especially badly, largelyunchecked, for more than a decade, and its malicious behavioris growing more egregious.
WannaCry was indiscriminatelyreckless." He said Washington must lead efforts to cooperate withother governments and businesses to "mitigate cyber risk andincrease the cost to hackers," and thus improve internetsecurity and resilience.
"When we must, the US will act alone to impose costs and consequences for cyber malfeasance," Bossert added.
President Donald Trump "has already pulled many levers ofpressure to address North Korea's unacceptable nuclear and missile developments, and we will continue to use our maximum pressure strategy to curb Pyongyang's ability to mount attacks, cyber or otherwise."
The WannaCry attack spread rapidly around the globe usinga security flaw in Microsoft's Windows XP operating system, anolder version that is no longer given mainstream tech supportby the US giant.
Ransomware, which can be used on PCs as well as tabletsand smartphones, is malicious software which locks computerfiles and forces users to pay the attackers a designated sumin the virtual Bitcoin currency to regain access to the files.
The Washington Post cited a US official as saying Trump's administration would be urging allies to counter North Korea's cyberattack capabilities and implement all "relevant" UN Security Council sanctions.
It said the CIA had already laid blame on North Korea forthe attack in November, though the assessment was classifiedand had not yet been previously reported.