Websites for the New York Stock Exchange, the US National Security Agency, the US Homeland Security department, and the Nasdaq stock market were also targeted, an early analysis of the malicious software used in the attacks has indicated.

Rich Mogull, the CEO of a cyber security firm, told Al Jazeera: "As far as we can tell, it looks like [the attack was carried out by] something known as a bot-net, which is a collection of computers under the control of a bad guy and has been directed to take down these sites.

"All those computers are being told 'let's team up on these targets and see if we can take them off the internet' ... Information suggests that this was a very crude form of this kind of attack."

White House 'unaffected'

The cyber-attack on the White House site had "absolutely no effect on the White House's day-to-day operations," Nick Shapiro, White House spokesman, said on Wednesday.

But while preventative measures kept the WhiteHouse.gov site "stable and available to the general public," he said, internet visitors from Asia may have experienced problems in getting access to the site.

As of Tuesday night, "all federal websites were back up and running," he said.

Websites of major South Korean government agencies, including the presidential Blue House and the Defence Ministry, were also the target of cyber-attacks.

South Korea's main spy agency told South Korean politicians on Wednesday that it believes North Korea or pro-North Korea groups carried out the attacks, according to an aide to one of the legislators who was at the briefing.

But cyber-warfare experts have said that it is too early to say who may have carried out the attacks.

"Even if you are right about the fact of being attacked, initial diagnoses are often wrong," Professor Peter Sommer, an expert on cyberterrorism at the London School of Economics, said.

The US Treasury's site was brought down on occasion during the attacks, two US government officials, speaking on condition of anonymity, said.

Ed Donovan, a spokesman for the US Secret Service, said that the cyber
attacks slowed down access to the agency's Web site.

"Our site was never knocked down, but it was slowed down at points," Donovan said, adding that Secret Service's operational side was not affected.