Google has confirmed that hundreds of attacks have been made on personal Gmail accounts of senior US government officials, political activists, military personnel and journalists.
Giving details of the hack in a blog post, Google suggests that the attempt to collect user passwords originated in Jinan, China.
"The goal of this effort seems to have been to monitor the contents of these users' email, with the perpetrators apparently using stolen passwords to change peoples' forwarding and delegation settings," reads the blog.
"Google detected and has disrupted this campaign to take users' passwords and monitor their emails. We have notified victims and secured their accounts. In addition, we have notified relevant government authorities.
"It's important to stress that our internal systems have not been affected – these account hijackings were not the result of a security problem with Gmail itself".
Security company Sophos suggests that government and other high profile officials should be more cautious about how they store sensitive information.
"One thing that people should be asking themselves, is why are they storing sensitive information in cloud based accounts in the first place?" said Graham Cluley, senior technology consultant at Sophos.
"The recent news headlines claim that senior US political and military officials were being targeted by these hackers, but surely they shouldn't be storing confidential or sensitive information in their webmail account," he added.
"Users should always think about the data that they are storing in their web email, because if it's only protected by a username and password, it may actually be less secure than your regular work email system."