A widespread computer attack has hit several US government agencies. Some South Korean government websites also appear to be affected.
The US Treasury Department, Secret Service, Federal Trade Commission and Transportation Department were all hit by the attack that started on July 4.
In South Korea, the presidential Blue House and Defence Ministry, National Assembly appear to have been hit, BBC News reports.
Unidentified hackers carried out massive so-called distributed denial-of-service (DDoS) attacks against the Web sites of the presidential office and other organizations Tuesday through Wednesday, causing them to shut or slow down for hours.
A DDoS attack involves sending large amounts of data that renders Web servers unusable by obstructing communication between the intended server and the target.
The incident came as a slap in the face to South Korea, which is touted as one of the most wired countries in the world, with about two-thirds of its 50 million population having high-speed Internet access, Yonhap News reports.
"The attacks appear to have been elaborately prepared and executed at the level of a group or a state," the National Intelligence Service (NIS) said in a statement.
It did not specify which group or state may be involved, Yonhap News reports.