The attack was also reported to have targeted Auction.com, the South Korean site of online auctioneer Ebay.

Ahn Jeong-eun, a spokeswoman for the government's Korea Information Security Agency, said the attacks appeared to be linked to an ongoing and unusually resilient attack against government websites in the US, including sites for the US treasury and the Secret Service.

The US attacks began on July 4, the US Independence Day holiday, and continued to affect access to several sites well into Tuesday.

Ahn said that initial investigations showed the attacks appeared aimed only at paralysing access to the sites, rather than hacking into secure networks or stealing information.

Some sites had returned to normal operation by midday on Wednesday, although access to others, including the site of the South Korean president and defence ministry, remained unobtainable.

Denial of service

The US and South Korean cases involve what are known as denial of service attacks, in which an unusually high number of computers all try to connect to a single website at the same time.

That deluge of requests overwhelms the server handling the traffic, in effect shutting it down.

Mounting such an attack can be relatively easy using widely available hacking programs, but they can be made far more serious if hackers infect and use thousands of computers – often without owners' knowledge - tied together into "botnets".

South Korea is one of Asia's most wired nations with two thirds of its population is connected to high-speed internet services.

Cyber command

There has been no indication as to who may be behind the South Korean attack, but it comes just weeks after the country's military announced it was setting up a special cyber warfare command to fend off attacks from North Korea and other countries.

According to a report released in May, South Korea's military networks experience an average of tens of thousands of hacking and virus attacks every day.

Recent years have seen governments around the world ramp up investment in cyber warfare and cyber defence as part of their security and military programmes.

Last year, in the weeks leading up to the war between Russia and Georgia, Georgian government and corporate websites began to experience a sudden string of "denial of service" attacks.

Russian government officials denied involvement, but after investigating domain names and website registration data, a group of independent Western computer experts concluded that the Russian security and military intelligence agencies were involved.