Some South Korean and US Internet sites were down or slowed to a crawl for a third day on Thursday after attacks by a hacker that Seoul's National Intelligence Service said may be linked to a cyber warfare unit in the North.

The impact of the attack was seen as largely symbolic and experts said it did not represent an actual security breach or damage to the online infrastructure of the world's most wired country.

South Korean media, including Yonhap news agency, quoted parliament members as saying after an intelligence briefing on Wednesday that the spy agency believed "North Korea or pro-North elements" were behind the attacks.

But some analysts raised doubts the North was responsible, saying the implications of such a state involvement were severe, and it may be the work of industrial spies or pranksters instead.

South Korea's Communications Commission said in a statement that it had stepped up counter measures after Wednesday's fresh wave of attacks, asking Internet service providers to filter access by computers infected with malicious software.

South Korea's defence ministry Web site was among those that remained down for a third day and access to some US government sites, including the State and Defence Department, from South Korea appeared to have been disabled.

An official with the Communications Commission said the impact could spread as more people go online through the day.

"We've had the first wave and the second wave, and now there may be a third one," Park Chul-soon said on state-run KBS radio.

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