South Korea's top games developer vowed Monday to tighten security after the personal information of 13 million users of its popular online game was leaked.

Nexon Korea apologised for the hacking last week, which revealed data including user IDs, names, passwords and residential registration numbers, on the subscribers to its game MapleStory.

The game has a global user base of 18 million but Nexon said no data on overseas users had been leaked.

"We are taking all measures to prevent possible damages from the leakage and will come up with follow-up measures to ease your anxiety," CEO Seo Min said in a statement on the firm's website.

He said Nexon would step up data protection by establishing global security control centres in key regions such as North America, Asia and Europe to monitor computer security around the clock.

Nexon said it would hire 30 more security experts, ask all users to change their passwords and introduce an integrated log-in process highly protected from hackers next year.

Police and telecom authorities are probing the case but it is still unknown who gained access to Nexon's server.

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.

In March last year authorities launched a probe into the security systems of major retailer Shinsegae and 24 other companies after private data on 20 million customers was leaked.

In July hackers gained access to major South Korean websites to steal the private information of 35 million users.

Earlier this year Japanese giant Sony was attacked in one of the biggest data breaches since the advent of the Internet, in which the user names, passwords, addresses and birth dates of more than 100 million people may have been compromised.

The company later suffered attacks on websites including in Greece, Thailand and Indonesia, and on the Canadian site of mobile phone company Sony Ericsson.