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

agency CISA says that Russian government-backed hackers have

used their access to Microsoft's ( MSFT ) email system to steal

correspondence between officials and the tech giant.

In an emergency directive released Thursday, the agency said

that the theft of emails between government officials and

Microsoft ( MSFT ) poses an "unacceptable risk to agencies."

It did not identify the agencies involved.