It follows a turbulent few months in which secretive North Korea has engaged in a series of threats and provocative actions widely condemned by the international community, including a nuclear test and missile launches.

North Korea has not responded to the allegations of its involvement in the Web site outages.

On Thursday, seven South Korean Web sites - one belonging to the government and the others to private entities - were attacked in the third round of cyber assaults, said Ku Kyo-young, an official from the state-run Korea Communications Commission, but most were back up and running quickly.

As previously, it was caused by so-called denial of service attacks, Ku said, adding that attacks were continuing on the seven sites Friday, but they were still accessible.

Some 50 cases of problems regarding computer hard disks or data were reported Friday morning in South Korea, the commission said, without giving details.

Park, the South Korean lawmaker, said Thursday that a senior intelligence official told her the NIS suspects the North because the country earlier warned it won't tolerate what it claimed were South Korean moves to participate in a US-led cyber warfare exercise, according to a statement from the opposition Democratic Party.

South Korean media reported in May that North Korea was running a cyber warfare unit that tries to hack into US and South Korean military networks to gather confidential information and disrupt service.

Japan was also being extra vigilant against possible cyber attacks, although there was no sign it had been targeted, officials said.

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