According to computer security specialist AhnLab, the massive denial of service (DoS) attack that took down some of South Korea's highest profile websites yesterday is set to resume this evening.

The attack will restart at 9am GMT (that's 6pm local time) and be directed at a smaller number of sites that those hit a day earlier. They will include government websites and the home pages of the Chosun Ilbo newspaper and Kookmin Bank.

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A DoS attack involves sending a massive volume of traffic to a web site so that it becomes overloaded. While some users will occasionally be able to access the site being attacked most will see nothing until a network time-out message appears.

Wednesday's attack hit some of the most popular sites in South Korea including the Chosun Ilbo, Internet Auction and the electronic banking services of several major banks. Government ministries and US Forces Korea were also taken offline by the attack that lasted throughout the working day.

See also: Massive botnet targets US government websites

The attack is being carried out by tens of thousands of computers infected with the MyDoom virus. Many of those PCs are in South Korea although the person or persons behind the attack are likely somewhere else, according to security experts.

Over the weekend the same network of PCs was used to attack major government and commercial websites in the US.

Due to the nature of the websites attacked some have pointed the finger at North Korea. Tensions between the country and the US and South Korea are currently high following several short range missile tests at the weekend and a recent nuclear test. However security experts say they have seen nothing to indicate the North Korean government is behind the attacks.