According to Microsoft on Wednesday, Chinese state-linked hackers secretly accessed email accounts at roughly 25 organizations, including government institutions.
White House national security advisor Jake Sullivan told ABC News that the US noticed a compromise of federal government accounts "fairly quickly" and was able to prevent subsequent breaches.
The hacking gang, Storm-0558, used counterfeit digital authentication tokens to access webmail accounts running on Microsoft's Outlook service, according to a company website statement.
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"As with any observed nation-state actor activity, Microsoft has contacted all targeted or compromised organizations directly via their tenant admins and provided them with important information to help them investigate and respond," the statement continued.
Microsoft did not specify which organizations or countries were compromised but did state that the hacker gang in question predominantly targets entities in Western Europe.
Without elaborating, White House National Security Council spokesperson Adam Hodge claimed a hack into Microsoft's cloud security had "affected unclassified systems."
"Officials contacted Microsoft immediately to find the source and vulnerability in their cloud service," he added.
The Chinese embassy in London did not immediately respond to an email seeking comment. Beijing frequently denies any role in cybercrime.
