SEOUL (Reuters) - South Korean authorities issued a cyber security warning on Wednesday after the websites of government agencies and financial institutions were disabled by apparent hacker attacks, possibly linked to North Korea.

South Korea's spy agency said in a statement an organization and possibly a state was behind the attacks on Tuesday in the world's most wired nation, and there were signs of "meticulous preparations" for the act.

Yonhap said the National Intelligence Service believes "North Korea or pro-North elements" were behind the hacking attacks, quoting parliament intelligence committee members.

The websites of the presidential office, defense ministry, and the National Assembly were saturated with access requests generated by malicious software on Tuesday, crippling server response to legitimate traffic, South Korea's Communications Commission said in a statement.

"The attacks consisted of massive harmful traffic to specific sites causing access slowdown or disablement, and some national institutions, banks and media sites have been targeted," it said.

The presidential Blue House said separation of its internal network from the Internet made it impossible for any hackers to gain access to classified information, but some parts of its websites remained out of service.

News of the attack pushed shares of some online security firms to jump on Wednesday, with Ahnlab Inc up by the 15 percent daily limit on the junior Kosdaq market, which ended trading down.

The attacks left some government websites and online shopping services down on Wednesday and access to some U.S. government sites from the country appeared to have been disabled.

The commission is working to block the spread of malicious software suspected of causing the attack and has advised users to keep security patches and anti-virus programmes up to date.

Police and prosecutors have begun an investigation into the incidents, South Korea's Yonhap news agency said.

A similar attack on major websites in Estonia two years ago prompted the NATO military alliance to review its response against possible "cyber-warfare."

Copyright 2009 Reuters. Click for restrictions.