WASHINGTON (Reuters) – Facebook Inc and Microsoft Corp disabled a number of North Korean cyber threats last week, a White House official said on Tuesday, as the United States publicly blamed Pyongyang for a May cyber attack that crippled hospitals, banks and other companies.

“Facebook took down accounts that stopped the operational execution of ongoing cyber attacks and Microsoft acted to patch existing attacks, not just the WannaCry attack initially,” White House homeland security adviser Tom Bossert said on Tuesday.

Bossert did not provide details on the actions by the two American tech heavyweights but said the U.S. government was calling on other companies to cooperate in cyber security defense.

Bossert’s remarks came during a White House news conference in which he blamed Pyongyang for the WannaCry attack that infected hundreds of thousands of computers in more than 150 countries, saying the U.S. government had clear evidence that North Korea was responsible. He did not share that evidence.

The U.S. accusation came at a time of high tension with North Korea over its nuclear weapons and missile programs.

A Facebook (FB.O) spokesman confirmed that the company last week deleted accounts associated with a North Korea-linked hacking entity known as Lazarus Group “to make it harder for them to conduct their activities.” The accounts were mostly personal profiles operated as fake accounts that were used to build relationships with potential targets, the spokesman said.

Facebook said it also notified individuals in contact with these accounts.

The actions echoed similar steps the social media…