Hackers using an Internet address registered in China have gained access to major South Korean websites and may have stolen the private information of 35 million users, authorities said Thursday. They breached the systems of Nate (www.nate.com) and Cyworld (www.cyworld.com), both run by SK Communications, the Korea Communications Commission said. Nate is a search engine with 25 million users and Cyworld is a social networking website with 33 million users in a country with a population of 48.6 million. From the two sites combined, information on about 35 million users such as names, web IDs, phone numbers, e-mail addresses and resident registration numbers appear to have been leaked, the commission said in a statement. SK Communications has asked police to investigate the cyber attack, commission official Kim Kwang-Su told AFP, adding his office was still investigating the incident. \"We have no information on who the hackers were. This could be the biggest hacking incident in our country,\" he said. The previous worst computer security breach was in February 2008, when Internet Auction -- a subsidiary of US firm eBay -- was hacked. This led to the theft of private information on more than 10 million users. \"We took security steps after detecting a malicious code originating from an IP (Internet protocol) address from China on Tuesday,\" said SK Communications spokeswoman Koo Ki-Hyang. \"Our probe is still under way but we believe information on an estimated 35 million users might have been leaked.\" South Korea, the world\'s most wired nation with more than 90 percent of homes connected to the Internet, has expressed concern about cyber attacks by Chinese and North Korean hackers. In 2004 hackers based in China allegedly used information-stealing viruses to break into the computer systems of Seoul government agencies. Seoul accused Pyongyang of staging cyber attacks on websites of major South Korean government agencies and financial institutions in March this year and in July 2009. In May South Korea said a North Korean cyber attack paralysed operations at one of its largest banks. North Korea reportedly maintains elite hacker units.