More than two dozen Internet sites in South Korea and the United States including the White House were attacked and some disabled by hackers possibly linked to North Korea, South Korea's spy agency said on Wednesday.
The National Intelligence Service (NIS) said in a statement an organisation and possibly a state were behind the attacks on Tuesday in South Korea, the world's most wired nation, and there were signs of "meticulous preparations" for the act.
The statement did not offer further details but South Korean media, including Yonhap news agency, quoted parliament members as saying after a briefing with NIS officials that the spy agency believes "North Korea or pro-North elements" were behind the attacks.
"Malicious programs" were found targeting 26 U.S. and South Korean Web sites, including that of the White House, NIS officials reportedly said, Yonhap reported.
The attackers tried to jam the web sites by overwhelming their data capacity and knocking them out of service, it said.
If the North was responsible, it would mark an escalation in tensions already high due to Pyongyang's nuclear test in May, its firing of a barrage of seven ballistic missiles in July and repeated taunts of long-time foes Seoul and Washington in its official media.
Access to the Internet is denied to almost everyone in impoverished North Korea, but intelligence sources in Seoul have said the secretive state has stepped up a unit that specialises in cyber attacks.
"MASSIVE HARMFUL TRAFFIC"
The web sites of the South Korea's presidential office, defence 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 sections of its web sites remained out of service.
News of the attack pushed shares of some online security firms higher on Wednesday, with Ahnlab Inc up the 15 percent daily limit on the junior Kosdaq market, which ended trading down.
The attacks left some government web sites and online shopping services down on Wednesday and access to some U.S. government sites from the country appeared to have been disabled, perhaps due to security measures in the United States that blocked access from South Korea due the attacks, experts told local media.
Police and prosecutors have begun an investigation into the incidents, South Korea's Yonhap news agency said.
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".