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".

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