A widespread and unusually resilient computer attack that began July 4 knocked out the Web sites of several government agencies, including some that are responsible for fighting cyber crime.

The Treasury Department, Secret Service, Federal Trade Commission and Transportation Department Web sites were all down at varying points over the holiday weekend and into this week, according to officials inside and outside the government. Some of the sites were still experiencing problems Tuesday evening. Cyber attacks on South Korea government and private sites also may be linked, officials there said.

US officials refused to publicly discuss details of the cyber attack. But Amy Kudwa, spokeswoman for the Homeland Security Department, said the agency's US Computer Emergency Readiness Team issued a notice to federal departments and other partner organizations about the problems and "advised them of steps to take to help mitigate against such attacks."

The US, she said, sees attacks on its networks every day, and measures have been put in place to minimize the impact on federal Web sites.

It was not clear whether other federal government sites also were attacked.

Others familiar with the US outage, which is called a denial of service attack, said that the fact that the government Web sites were still being affected three days after it began signaled an unusually lengthy and sophisticated attack. The officials spoke on condition of anonymity because they were not authorized to speak on the matter.

Web sites of major South Korean government agencies, banks and Internet sites also were paralyzed in a suspected cyber attack Tuesday. Ahn Jeong-eun, a spokeswoman at the Korea Information Security Agency, said the US and South Korean attacks appeared to be linked.

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