The UK was the likely source of a series of attacks last week that took down popular websites in the US and South Korea, according to an analysis performed by a Vietnamese security analyst.

The results contradict assertions made by some in the US and South Korean governments that North Korea was behind the attack. Security analysts had been skeptical of the claims, which were reportedly made in off-the-record briefings and for which proof was never delivered.

The week-long distributed denial of service attack involved sending multiple requests to a handful of websites from tens of thousands of computers so the sites became overloaded.

Among the sites taken offline at some time during the week were those of the US Departments of Transportation and Treasury, the US Federal Trade Commission, the South Korea's president's home page, the South Korean national assembly and US Forces Korea.

The computers used to send the flood of requests had been infected with a virus that allowed attackers to use them anonymously.

Every three minutes the infected computers randomly selected one of eight servers to connect to and receive orders, said Nguyen Minh Duc, senior security director at Bach Khoa Internetwork Security (Bkis), in a blog posting on the company's web site. Bkis says it gained control of two of the eight servers and through this has been able to discover the master server.

That server has an IP address in the 195.90.118.x range, Nguyen said.

The address is registered to Global Digital Broadcast in the UK. The company could not immediately be contacted.

"Having located the attacking source in UK, we believed that it is completely possible to find out the hacker," Nguyen wrote.

Through analysing the log files of the two servers it controls, Bkis said the attacks utilized 166,908 PCs in 74 countries that had been infected. That figure is significantly higher than the "several tens of thousands" that other security companies had estimated were involved.

The largest number of infected PCs were in South Korea followed by the US, China, Japan, Canada, Australia, the Philippines, New Zealand, the UK and Vietnam.