WASHINGTON, April 11 (Reuters) - The U.S. Cybersecurity

and Infrastructure Security Agency said Russian

government-backed hackers have used their access to Microsoft's ( MSFT )

email system to steal correspondence between officials

and the tech giant, an emergency directive by the U.S. watchdog

released on Thursday showed.

In the directive dated April 2, the agency warned that

hackers were exploiting authentication details shared by email

to try to break into Microsoft's ( MSFT ) customer systems, including

those of an unspecified number of government agencies.

The warning that government agencies are being targeted

using stolen Microsoft ( MSFT ) emails follows the company's announcement

in March that it was still wrestling with the intruders, which

it nicknames "Midnight Blizzard."

That disclosure, which set alarm bells ringing across

the cybersecurity industry, was followed just last week by a

report from the U.S. Cyber Safety Review Board which said that a

separate hack - blamed on China - had been preventable,

faulting the company

for cybersecurity lapses and a deliberate lack of

transparency.

CISA declined to name agencies that might have been

affected. Microsoft ( MSFT ) said in an email that it was "working with

our customers to help them investigate and mitigate. This

includes working with CISA on an emergency directive to provide

guidance to government agencies."

The Russian Embassy in Washington, which in the past has

denied being behind hacking campaigns, did not immediately

return a message seeking comment.

CISA warned that the hackers might have gone after

non-governmental groups as well.

"Other organizations may also have been impacted by the

exfiltration of Microsoft ( MSFT ) corporate email," CISA said,

encouraging customers to contact Microsoft ( MSFT ) for further details.