As expected, South Korea suffered a third wave of online attacks at the end of last week, as malware-infected PCs were reactivated in a major botnet that launched attacks on government banking and media sites, according to new reports.

The Washington Post said that the distributed denial-of-service (DDoS) attacks, which struck on Thursday evening, were much more widespread than at first predicted. Around six government sites, including those of parliament, the Defense Ministry and the Foreign Ministry, were slowed down or temporarily stopped from working.

The MyDoom virus was once again the root cause of the attacks, turning PCs into zombie computers capable of launching a co-ordinated DDoS attack on any sites selected by the hackers.

Although there were no major security breaches as a result of the attacks, there could be a new phase which targets data on tens of thousands of PCs, according to a Daily Telegraph report.

Lee Byung-cheol, of South Korean web security firm Ahnlab, is quoted as saying: "The affected computers will not be able to boot and their storage files will be disabled."

North Korea is not among the five countries listed by the Korean Communications Commission (KCC) where the attack could have originated, said the report.

The KCC said that web sites believed to be behind the attacks were actually hosted in Germany, Austria, Georgia, the US and South Korea, although the location of the hackers is unknown and North Koreans could still be responsible.