Computer systems of several government departments in the United States have been disrupted by a concerted cyber attack.
The websites of the Treasury, the Secret Service, the Federal Trade Commission, and the Transportation Department were all affected, the BBC reports.
Officials say it was an unusually lengthy and sophisticated operation.
In South Korea, the presidential Blue House, the Defence Ministry, the National Assembly and several banks have also been hit by a cyber attack.
South Korea's Information Security agency says the attacks appear to be connected to those in the US.
North Korea's Yonhap News Agency is reporting North Korea may be behind the cyber attacks.