The US is expressing growing concern over a hack on Microsoft's Exchange email software that the tech company has blamed on China.
"This is an active threat," White House press secretary Jen Psaki said on Friday. "Everyone running these servers - government, private sector, academia - needs to act now to patch them."
Microsoft said hackers had used its mail server to attack their targets.
It is reported that tens of thousands of US organisations may be impacted.
The US has long accused the Chinese government of cyber-espionage, something Beijing denies.
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