SEOUL, SOUTH KOREA: South Korean authorities issued a cyber security warning on Wednesday after the Web sites of several major government agencies and financial institutions were disabled by apparent hacker attacks.

The Web sites 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.

Some government Web sites and online shopping services remained 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 programmers up to date.

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

News of the attack caused shares of some online security firms to jump on Wednesday morning, with Ahnlab Inc up by the market's 15 percent daily limit and ESTsoft Corp climbing 5.31 percent against the junior Kosdaq market 0.36 percent slide.

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