Chinese hackers relating to Chinese state they secretly gained access to the electronic accounts of about 25 organizations including US government agencies in a sweeping cyber espionage attackannounced today Microsoft.

In an interview with ABC television, White House national security adviser Jake Sullivan said the United States “relatively quickly” detected breaches in federal government accounts and took steps to prevent further breaches.

The hacking group, which Microsoft named “Storm-0558,” forged digital authentication tokens to gain access to webmail accounts running on the company’s Outlook service, Microsoft said in a statement on its website. Its activity began in May.

“As with any observed activity by a government actor, Microsoft contacted all targeted or exposed organizations directly through their administrators and provided them with important information to help them investigate and respond,” the statement added.

Microsoft did not say which organizations or government agencies were affected by the hacks, but added that the group of hackers primarily targeted entities in Western Europe.

White House National Security Council spokesman Adam Hodge said the breach in Microsoft’s cloud security affected “unclassified systems,” without elaborating.

“Officials immediately contacted Microsoft to find the source and variability in the cloud service,” he added.

China’s embassy in London did not immediately respond to an emailed request for comment. Beijing has consistently denied involvement in hacking.