"We recently uncovered a campaign to collect user passwords, likely through phishing," Google said, making a reference to an email hacking practice where perpetrators try to trick users into handing over their passwords.

"This campaign, which appears to originate from Jinan, China, affected what seem to be the personal Gmail accounts of hundreds of users including, among others, senior U.S. government officials, Chinese political activists, officials in several Asian countries (predominantly South Korea), military personnel and journalists," Google said.