Google Inc. pioneered the practice in 2012 and said it now alerts tens of thousands of users every few months.

For two years, Microsoft has offered alerts about potential security breaches without specifying the likely suspect.

In the statement, Microsoft said: "As the threat landscape has evolved our approach has too, and we'll now go beyond notification and guidance to specify if we reasonably believe the attacker is 'state-sponsored'."

Microsoft declined to say what role, if any, the Hotmail hacking campaign played in its policy change.

The Hotmail attacks had also targeted diplomats, media workers, human rights lawyers, and others in sensitive positions inside China, according to the former employees.

Microsoft had told the targets to reset their passwords but did not tell them that they had been hacked. Five victims interviewed by Reuters said they had not taken the password request as an indication of hacking.

Online free-speech activists and security experts have long called for more direct warnings, saying that they prompt behavioral changes from email users.