The U.S. has declared North Korea as the perpetrator of a ransomware cyberattack that infected hundreds of thousands of computers worldwide in May.

The cyberattack was rapid, and in no time hit hospitals, companies, and other critical institutions worldwide, particularly Britain's National Health Service.

In a Wall Street Journal op-ed published on Monday, Homeland Security Adviser Tom Bossert said that North Korea was "directly responsible" for the WannaCry ransomware.

He added that the consequences and repercussions of the attack were beyond economic. Bossert reiterated WannaCry's impact on U.K.'s health care, after it compromised several systems critical to the sector. "These disruptions put lives at risk," he wrote.

Bossert clarified that the allegations were not made lightly. They were based on evidence that have already been confirmed by other countries like the United Kingdom, as well private companies like Microsoft.

Bossert also noted that Pyeongchang will be held accountable for the cyberattack.

"North Korea has acted especially badly, largely unchecked, for more than a decade, and its malicious behavior is growing more egregious. WannaCry was indiscriminately reckless," he wrote in the op-ed.

He hinted that the U.S. government will increase security measures against cyberattack, as well us step up its monitoring of North Korea when it comes to hacking.

"We will continue to use our maximum pressure strategy to curb Pyongyang's ability to mount attacks, cyber or otherwise," Bossert wrote.

In May, the WannaCry ransomware locked up digital documents and databases across 150 countries, and demanded for a ransom so they could be released.

It particularly exploited a vulnerability in many older versions of Microsoft Windows. Computers that were affected are those that have not been patched with the latest security fixes that otherwise could have blocked the cyberattack.

In June, the National Security Agency's initial investigation already pointed out to North Korea as the perpetrator, after cybersecurity researchers saw code similarities between WannaCry and a malware previously traced back to North Korea.