June 27 (Reuters) - Russian hackers who broke into

Microsoft's ( MSFT ) systems and spied on staff inboxes earlier

this year also stole emails from its customers, the tech giant

said on Thursday, around six months after it first disclosed the

intrusion.

The disclosure underscores the breadth of the breach as

Microsoft ( MSFT ) faces increasing regulatory scrutiny over the security

of its software and systems against foreign threats. An

allegedly Chinese hacking group that separately breached

Microsoft ( MSFT ) last year stole thousands of U.S. government emails.

The Russian government has never responded to the Microsoft ( MSFT )

hacking allegations, but Microsoft ( MSFT ) has said the hackers targeted

cybersecurity researchers who had been investigating the Russian

hacking group's actions.

"This week we are continuing notifications to customers who

corresponded with Microsoft ( MSFT ) corporate email accounts that were

exfiltrated by the Midnight Blizzard threat actor," a Microsoft ( MSFT )

spokesperson said in an emailed statement. Bloomberg first

reported on the action earlier in the day.

Microsoft ( MSFT ) said it was also sharing the compromised emails

with its customers, but did not say how many customers had been

impacted, nor how many emails may have been stolen.

"This is increased detail for customers who have already

been notified and also includes new notifications," the

spokesperson said. "We're committed to sharing information with

our customers as our investigation continues."

Back in January, the world's largest software vendor had

said that Midnight Blizzard had accessed "a very small

percentage" of the company's corporate email accounts. Four

months later it said those hackers were still trying to break

in, alarming many of its security industry peers and customers

who questioned why Microsoft's ( MSFT ) systems remained vulnerable.

Those intrusions, and the Chinese hack last year, prompted a

Congressional hearing earlier this month where Microsoft ( MSFT )

President Brad Smith said the company was working on overhauling

its security practices.