If the North was responsible, it would mark an escalation in tensions already high from Pyongyang's nuclear test in May, a barrage of ballistic missiles in July and repeated taunts of long-time foes Seoul and Washington in its official media.

SEVERE IMPLICATIONS

Mark Rasch of SecureITExperts, a former US Department of Justice official on cyber crimes, said the implications of a state sponsored attack were severe.

"There's no difference between dropping a logic bomb and dropping a TNT bomb in the law of war," he said, but added that while North Korea could have been behind the maneuvers, they did not appear to be coming from computers physically based there.

Last month the North warned of "high-tech war" against the South for spreading what it said was false information about its involvement in cyber attacks.

"As universally recognised, the US is the kingpin of 'cyber attack' and 'hacker intrusion' on our planet," its official KCNA news agency said on June 27.

"The DPRK is fully ready for any form of high-tech war," it said using the country's official name, the Democratic People's Republic of Korea.

An expert on the North at the Heritage Foundation, Bruce Klingner, said the North had in operation a military unit with up to 1,000 skilled computer hackers created 10 years ago.

"Pyongyang has an extensive and capable cyber terrorism effort to provide asymmetric attack capabilities," he said.

Internet security shares were up sharply on Thursday for a third day led by AhnLab, which rose by nearly the junior Kosdaq market's daily limit of 15 percent.

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