A widespread and unusually resilient computer attack that began on July 4 knocked out the websites of several US government agencies - including some responsible for fighting cyber crime, it emerged today.

The Treasury Department, Secret Service, Federal Trade Commission and Transportation Department websites were down at varying points over the Independence Day holiday weekend and into this week, according to officials inside and outside the government.

Some of the sites were still experiencing problems early today.

Government officials refused to publicly discuss any details of the cyber attack, and would only generally acknowledge that it occurred. It was not clear whether other government sites also were attacked.

Others familiar with the outage, which is called a denial of service attack, said that the fact that the government websites were still being affected three days after it began signalled an unusually lengthy and sophisticated attack.

The Homeland Security Department confirmed that officials had received reports of "malicious web activity" and they were investigating the matter, but had no further comment.

Two government officials acknowledged that the Treasury and Secret Service sites were brought down and said the agencies were working with their internet service provider to resolve the problem.

Ben Rushlo, director of internet technologies at Keynote Systems, called it a "massive outage" and said problems with the Transportation Department site began on Saturday and continued until Monday, while the FTC site was down Sunday and Monday.

Keynote Systems is a mobile and website monitoring company which publishes data detailing outages on websites, including 40 government sites it watches.

According to Mr Rushlo, the Transportation website was "100% down" for two days so that no internet users could get through to it. The FTC site, meanwhile, started to come back online late on Sunday, but even early today users still were unable to get to the site 70% of the time.

"This is very strange. You don't see this," he said. "Having something 100% down for a 24-hour-plus period is a pretty significant event."

Denial of service attacks against websites are not uncommon and are usually caused when sites are deluged with internet traffic to effectively take them offline.

Mounting such an attack can be relatively easy using widely available hacking programs, and they can be made far more serious if hackers infect and use thousands of computers tied together into "botnets".

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 Western computer experts traced domain names and website registration data to conclude the Russian security and military intelligence agencies were involved.

:: South Korea said today the websites of major government agencies, banks and internet sites were paralysed because of a suspected cyber attack.

The state-run Korea Information Security Agency said websites of the presidential Blue House, the Defence Ministry, the National Assembly, Shinhan Bank, Korea Exchange Bank and top internet portal Naver had been down or had access problems.

Some of the sites remained unstable today.

Agency spokeswoman Ahn Jeong-eun said the paralysis happened because of a deluge of computers trying to connect with the affected sites, overwhelming computer servers that handle the traffic.