It was the second time in the last two weeks that Google has deployed security-related alerts to a small fraction of those who use its services.

But the company was coy about how it knows whether a specific individual has been targeted by attacks paid for or designed by governments.

"You might ask how we know this activity is state-sponsored," said Eric Grosse, Google's vice president of security engineering, in a . "We can't go into the details without giving away information that would be helpful to these bad actors."

The new warning states: "We believe state-sponsored attackers may be attempting to compromise your account or computer." It will appear at the top of the Gmail page if the user has logged in with his or her Google account. The message is not limited to those who use Google's own Chrome, but will pop up in any browser.

Grosse was equally vague about what might trigger the alert.