Government ministries, the National Assembly, the military headquarters, US Forces in the RoK and major banks were among those hit.

It is believed that the attackers injected malware into two peer-to-peer file-sharing websites.

The attacks are similar to those that targeted South Korean websites in July 2009.

Some 29 institutions were affected by so-called distributed denial-of-service attacks (DDoS) which overload a site with data causing it to fall over.

The web page of the Financial Services Commission, the country's financial regulator, was overloaded and an online stock trading system was shut down for a few minutes but both soon recovered, according to government sources.

"There was a DDoS attack, but no damage was done," said an official from the presidential office.

The RoK’s security firm AhnLab expected another wave of attacks on Friday, targeting up to 40 government and corporate websites.

It estimates that up to 11,000 personal computers were infected by malware and recruited for the attack. It is distributing free software to clean PCs.

The RoK’s cyber investigation unit has sent investigators to the two file-sharing sites that are believed to have spread the malicious code, according to the National Police Agency.

The cyber attacks against the RoK in 2009 were blamed on the Democratic People’s Republic of Korea (DPRK), although no link has been proven.

The RoK’s media outlets have, in the past, accused the DPRK of running an internet warfare unit aimed at hacking into US and the RoK’s military networks.