The websites of several US government agencies, including those responsible for fighting cyber crime, were knocked out by a widespread and unusually resilient computer attack.

The treasury department, secret service, and federal trade commission sites were all down at varying points over the Independence Day holiday weekend and into this week.

Cyber attacks on South Korean government and private sites may also be linked. Amy Kudwa, spokeswoman for the homeland security department, said the agency's US computer emergency readiness team issued a notice to federal departments to warn them “of steps to take to help mitigate against such attacks”.

Sources said the “denial of service” attack was lengthy and sophisticated.

Ahn Jeong-eun, a spokeswoman at the Korea information security agency, said the US and South Korean attacks appeared to be linked. The South Korean sites included a presidential one, the defence ministry, the national assembly and two banks.

Last summer, in the weeks leading up to the war between Russia and Georgia, Georgian government and corporate websites began to see “denial of service” attacks. The Kremlin denied involvement, but a group of independent computer experts concluded that Russian security and military intelligence agencies were involved.

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