By John Sudworth
BBC News, Seoul
This is the third wave of attacks on corporate websites in South Korea
South Korea is experiencing a third wave of suspected cyber-attacks - co-ordinated attempts to paralyse a number of major websites.
One of the country's biggest banks, a leading national newspaper and the South Korean spy agency appear to have been targeted.
Some reports suggest the attacks might be the work of North Korea.
South Korea and the US reported similar attacks earlier in the week, with the White House and the Pentagon targeted.
The South Korean government, and the country's internet service providers, are still trying to fight off what appears to be a deliberate attempt to shut down major websites that began earlier this week.
In what is known as a "denial of service" attack, thousands of virus-infected computers are hijacked and simultaneously directed to a particular site, overwhelming it with the sheer volume of traffic.
A third wave of attacks is now reported to be underway, slowing down or paralysing the internet operations of large organisations including a bank, a national newspaper and the South Korean spy agency.
The agency is reportedly the source of the speculation that the operation may have been instigated by North Korea or its sympathisers.
The United States is the only other country to have been affected by the cyber attacks, where the White House, the Pentagon and the State Department are all said to have been targeted.
While undoubtedly causing inconvenience for the organisations, and their customers, the attacks affect only their public websites, and present no other security threat.
Despite the speculation, no evidence has been produced to prove a link with North Korea.
But some reports suggest it has a long-established military unit, employing up to 1,000 skilled computer hackers.
In response to the ongoing disruption, South Korea has announced that it will speed up plans for a cyber warfare unit to counter such threats.