Chinese state-linked hackers secretly accessed email accounts at around 25 organizations, including government agencies, in a sweeping cyberespionage campaign by a group which primarily targets entities in Western Europe, Microsoft said on Wednesday.

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

“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 added.

Microsoft did not say which organizations – or governments -had been affected by the digital intrusion.

White House National Security Council spokesman Adam Hodge said in a statement on Wednesday that an intrusion in Microsoft’s cloud security had “affected unclassified systems,” without elaborating.

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

China’s Embassy in London did not immediately respond to an emailed request for comment. Beijing routinely denies that it engages in state-backed hacking campaigns.