North Korea is the main suspect behind a campaign of cyber attacks that have paralysed the websites of US and South Korean government agencies, banks and businesses since American Independence Day.

News of the campaign — which would be the first such large-scale attack attempted by the dictatorship — emerged yesterday as an ill and emaciated Kim Jong Il made a rare public appearance.

US government agencies, ranging from the Treasury Department to the Secret Service, and South Korea’s presidential office, parliament and defence and foreign ministries were among those hit by the attacks, which began on July 4.

The South Korean intelligence agency told members of parliament that it believed Pyongyang or its agents abroad were behind the attacks.

“This is not a simple attack by individuals,” Seoul’s National Intelligence Service (NIS) said in a statement. “The attack appeared to have been elaborately prepared and staged by a certain organisation or state.”

The Yonhap news agency quoted an unnamed member of parliament who said that the NIS had suggested that the attacks were the work of North Korea or “a pro-North Korean force”.

Experts said that there was no indication of data theft, but because the websites were still affected four days after the attacks began, an unusually sophisticated denial of service attack had probably been used. Thousands of computers were infected by a virus that flooded websites with traffic, then overloaded their servers and forced them to shut down.

South Korean and US authorities were working to identify the creators of the virus, the NIS said. US officials would not speak publicly about the attack but an emergency notice has been sent to federal departments advising them of precautions.

One theory is that they were tests of American and South Korean cyber defences. Security experts warned last year, amid an attack linked to China, that America remained more vulnerable to cyber-terrorism than any conventional form of attack.

President Obama announced the establishment of an inter-agency cyber-security office in May and the Pentagon added a military Cyber Command to supplement the civilian effort last month.

South Korea had warned of the danger of cyber-espionage by Chinese and North Korean hackers. The country’s Defence Security Command said last month that it was logging attempts to penetrate military networks at an average rate of 95,000 a day.

If the attacks this week are the work of North Korea the timing is significant. Pyongyang has staged a series of verbal and physical provocations this year, including an underground nuclear test and the launch of an intercontinental rocket.

The aggressive attitude suggests that it has abandoned expectations of negotiation with the international community in favour of whipping up nationalist fervour at home.

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