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

A denial of service 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 U.S. Forces Korea were also taken offline by the attack that lasted throughout the working day.

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 Web sites in the U.S.

Due to the nature of the Web sites attacked some have pointed the finger at North Korea. Tensions between the country and the U.S. 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.