The United States on Monday acknowledged for the first time that North Korea had created the WannaCry ransomware that affected more than 2,30,000 computers in over 150 countries in May.

“The attack was widespread and cost billions, and North Korea is directly responsible,” United States President Donald Trump’s Homeland Security Adviser Thomas P Bossert wrote in an op-ed published in The Wall Street Journal on Monday. “We do not make this allegation lightly. It is based on evidence.”

Bossert said that the US government was not the only one to reach this conclusion. “The United Kingdom attributes the attack to North Korea, and Microsoft traced the attack to cyber affiliates of the North Korean government,” he wrote.

On May 15, days after the attack that crippled computer systems worldwide cybersecurity companies Symantec and Kaspersky Lab claimed that they had found evidence that proved that North Korean cybergang Lazarus Group was behind the cyberattack. A month later, the US National Security Agency said that its analysis of the tactics, techniques and targets suggested that the the Reconnaissance General Bureau, North Korea’s spy agency, was behind the attack.

Bossert called on governments and businesses to cooperate to mitigate cyber risk. “The US must lead this effort,” he said, “rallying allies and responsible tech companies throughout the free world to increase the security and resilience of the internet.”

The Trump administration would now ask “all responsible states” to counter North Korea’s ability to conduct cyberattacks and implement United Nations Security Council sanctions against the country, The Washington Post quoted unidentified US officials as saying.