WASHINGTON — The Trump administration formally accused North Korea on Monday night of creating the WannaCry cyberattack that briefly paralysed the British health system and placed ransomware on computers in dozens of countries around the world.

President Donald Trump’s homeland security adviser, Thomas P. Bossert, wrote in an op-ed published in The Wall Street Journal that the conclusion was “based on evidence” that he did not disclose. He suggested that the United States would act against North Korea — beyond the sanctions now being imposed for its rapid expansion of its nuclear weapons program, writing, “When we must, the US will act alone to impose costs and consequences for cyber-malfeasance.”

The conclusion was not by itself a surprise: The New York Times reported in May that North Korea was the leading suspect in the attack, which encrypted hard drives on hundreds of thousands of computers and demanded the payment of ransom to unlock the data. Later, British officials reported that their forensic evidence and intelligence pointed to Pyongyang, and Bossert wrote that in recent days, Microsoft and other companies that track major threats have closed pathways that the North’s army of hackers could use for similar breaches.

Still, the assertion by the administration, which officials said would be supported at a White House news conference Tuesday, was notable for three reasons.

It came almost three years to the week since President Barack Obama, appearing in the White House press room, accused North Korea’s leadership of mounting a similarly sophisticated cyberattack on Sony Pictures Entertainment. The North was enraged that Sony was releasing a comedy, “The Interview,” envisioning a CIA-ordered assassination of Kim Jong Un, the country’s young leader. That was the first time the United States had accused another nation of a direct, destructive online intrusion on an American target.

But the decision to name the North also stands in stark contrast to how Trump has dealt with evidence that Russian hackers, under orders from President Vladimir Putin, organised the attack on the Democratic National Committee and the information warfare campaign that was meant to influence the 2016 election. Trump has often dismissed the intelligence finding that Russia was behind the hacking, declaring last month, “Putin said he did not do what they said he did.”

It is the same intelligence agencies — and some of the government’s same experts — that built the case against North Korea, according to members of the intelligence community who spoke on the condition of anonymity to discuss the investigation.

But the third, and perhaps most delicate, element of the WannaCry attack revolves around a fact that Bossert did not address in his op-ed: The North exploited vulnerabilities in software developed by the division of the National Security Agency that builds the United States’ cyberweapons. The code pulled off networks and computers compromised by WannaCry appears to have its roots in what the agency formerly called the Tailored Access Operations division, which devised online breaches.

Once it was clear the code had been stolen, the National Security Agency rushed to contain the damage, asking Microsoft to build a “patch” in its operating systems to prevent the attacks. But the agency has never talked about the group that stole the computer code, called the Shadow Brokers, which many officials believe is operating on behalf of the Russian government. But Bossert and his deputy, Rob Joyce, who formerly ran the Tailored Access Operations, have argued that it is the perpetrator of the attacks, not the US government, that must take all of the responsibility for the damage it has wreaked.

“The consequences and repercussions of WannaCry were beyond economic,” Bossert wrote. “The malicious software hit computers in the U.K.'s health care sector particularly hard, compromising systems that perform critical work. These disruptions put lives at risk.”

The assertion by the White House came only hours after Trump published his new national security strategy, which calls for pushing back on states that sponsor cyberactivity. And even some alumni of the Obama administration now agree that they often underreacted to a range of digital threats, including Iran’s 2012 attacks on American banks, the hacking at Sony and the effort by Russia to intervene in the election. Until now, North Korea’s cyberstrikes have prompted almost no punishment.

Bossert seems determined to change that, and he wrote about elements of a new digital strategy that suggests that the Trump administration will be more aggressive in alerting manufacturers to flaws found in their software. But he has been vague about what kind of actions might be taken against those who initiate cyberattacks.

Robert Hannigan, former director of Britain’s Government Communications Headquarters, said last month that in the realm of digital breaches, North Korea had benefited from being underestimated.

“Because they are such a mix of the weird and absurd and medieval and highly sophisticated, people didn’t take it seriously,” he said. “How can such an isolated, backward country have this capability? Well, how can such an isolated backward country have this nuclear ability?”

David E. Sanger/The New York Times