A widespread computer attack has hit several US government agencies while 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.
US officials have not released details of the attack.
Ben Rushlo, head of internet technologies at web performance firm Keynote Systems described it as a "massive outage".
Amy Kudwa, a spokeswoman for the Department of Homeland Security said the body's US Computer Emergency Readiness Team told federal departments about the issue and of steps "to mitigate against such attacks".
Recently the US homeland security secretary Janet Napolitano told the BBC that protecting against virtual attacks was a matter of "great concern" and something the US was "moving forward on with great alacrity".
The attacks in South Korea seemed to be connected to the attack of US government services, said Ahn Jeong-eun, a spokesperson at Korea's Information Security Agency.
North Korea's Yonhap News Agency is reporting that North Korea may be behind Tuesday's cyber attack.
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