Distributed denial-of-service attacks were directed at 29 South Korean Government websites including those hosted for the President late last week.
Sites affected included those hosted by US forces, the military Joint Chiefs of Staff, the ministries of foreign affairs, defence and unification, parliament and the tax office.
The website of South Korean IT security company AhnLab warned visitors that attacks were aimed at its domestic site and system damage caused by malicious code had been discovered.
The incidents began at 10:00am local time and the Korean Government said it was working with internet security agencies and others to deal with the problem.
An official from the presidential office told BBC News that "there was a DDoS attack, but no damage was done".
In in July 2009, attacks against South Korea and the US were blamed on North Korea.
In that instance, there were no reports of financial damage or data breaches and the alleged attacks seemed aimed to paralyse websites.