The US recently expressed concerns about cyber security
A widespread computer attack has hit several US government agencies while some South Korean government websites also appear to be affected.
The White House, Defense Department and New York Stock Exchange were all hit by the attack that started on July 4.
An analysis of the software used revealed its targets also included the National Security Agency, the Nasdaq stock market and the Washington Post.
Many of the organisations appear to have warded off the attack.
In South Korea, the presidential Blue House and Defence Ministry, National Assembly fell victim.
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.
South Korea's Yonhap News Agency is reporting that North Korea may be behind Tuesday's cyber attack.
The country's National Intelligence Service (NIS) suspects North Korea or its sympathisers may have been behind the attack, according to sources who spoke to the news agency on condition of anonymity.
It will present a report to the parliamentary intelligence committee on Thursday.
The attack slowed down and, in some cases, shut government websites, including the site of the presidential office, for several hours.