The personal information of about 35 million internet users in South Korea has been stolen in an alleged hacking attack that originated in China, officials have said.
South Korean police have begun an investigation.
Hackers attacked popular internet and social media sites Nate and Cyworld earlier this week, stealing data such as social security numbers and email addresses, the Korea Communications Commission said in a statement.
The regulator said that operator of the sites, SK Communications, alleged the attack originated from computers in China based on their internet Protocol addresses.
IP addresses are the web equivalent of a street address or phone number.
The stolen data included user IDs, passwords, social security numbers, names, mobile phone numbers and email addresses. Nate said the social security numbers and passwords are encrypted.
South Korea has faced internet attacks before, with blame frequently pinned on hackers operating from China. China has denied all charges of hacking in the past.
In May, South Korean prosecutors said that hackers in North Korea had broken into the computer network of a South Korean bank earlier this year.
North Korea has flatly denied any responsibility.