The White House, State Department and Pentagon websites were among those targeted in a coordinated cyber-attack which also crippled sites in South Korea, computer security experts said.

The Department of Homeland Security confirmed that US government and private sector websites had come under so-called “distributed denial of service” (DDoS) attack, but declined to identify any of the targeted sites.

A DDoS attack attempts to paralyse a website by overwhelming it with traffic from an army of malware-infected computers known as a “botnet”, but does not involve any theft of data.

The chief technology officer for the private SANS Internet Storm Centre, Johannes Ullrich, said the internet assault, which began over the weekend, was “a pretty massive attack”.

“Nothing really terribly sophisticated. It just floods the websites,” he said.

“It prevents the websites from responding. They’re just overloaded with traffic.

“The only site that was hit pretty bad was the Federal Trade Commission, ftc.gov.”

Mr Ullrich said the US government sites, which came under attack, included the White House, Department of Homeland Security, Department of Transportation, Federal Aviation Administration, National Security Agency, State Department, US Postal Service, US Treasury Department and Voice of America.

He said a Pentagon site, defenselink.mil, was also targeted, as was a site for US forces in South Korea.

North Korea ‘responsible’

South Korean politicians were quoted as saying that South Korea’s intelligence service believes North Korea or its sympathisers may have staged the attack.

Seoul’s National Intelligence Service (NIS) said in a statement that this was not a simple attack by individuals.

“The attack appeared to have been elaborately prepared and staged by a certain organisation or state,” it said.

The global intelligence network director at security firm Symantec, Dean Turner, said a “little over 50,000 machines” may have made up the botnet and the number of US and South Korean websites targeted was “greater than 20″.

But Mr Turner cautioned against assigning blame.

“We don’t know who is behind this, and we don’t necessarily know what the purpose is either,” he said.

“We can say where the attacks are coming from, where those machines are located but that doesn’t give us any visibility into the ‘who.’

“The person or persons responsible could be located on the moon.”

Mr Turner said if the attackers’ goal was to make these sites unavailable for an extended period of time, or take them off the internet, then it was “terribly successful”.

Private sector sites, which came under attack, included the New York Stock Exchange, the Nasdaq electronic exchange, Web portal Yahoo!, online retail giant Amazon and The Washington Post.

- AFP