Chinese state-linked hackers secretly accessed email accounts at around 25 organizations, including government agencies, in a sweeping cyberespionage campaign, Microsoft said Wednesday.

Microsoft did not say which organizations or governments had been affected by the digital intrusion, but the White House said the U.S. had reported it to Microsoft.

National Security Council spokesman Adam Hodge said in a statement on Wednesday that an intrusion in Microsoft's cloud security had "affected unclassified systems."

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

"We detected it fairly rapidly and we were able to prevent further breaches. The matter is still being investigated so I have to leave it there," National Security Adviser Jake Sullivan said on ABC's "Good Morning America."

The number of U.S. email accounts believed to be affected so far is limited, and the attack appeared targeted, The Washington Post reported, citing an unnamed source. Pentagon, intelligence community, and military email accounts did not appear to be affected, the Post rerported.