South Korea issued a cyber security alert after the websites of 40 government and other agencies came under attack today, the Korea Communications Commission (KCC) said.

A KCC spokesman confirmed details of the DDoS (Distributed Denial of Service) attacks given earlier by private Internet security firm AhnLab.

AhnLab said sites included those of the presidential Blue House, the US forces, the military Joint Chiefs of Staff, the ministries of foreign affairs, defence and unification, the spy agency, parliament and the tax office.

AhnLab said its own website had also come under attack along with those of seven major banks in the incident that began at 10 am (0100 GMT).

The intensity of the attacks was not immediately clear.

Some of the websites could be accessed around one hour later.

The KCC said in a statement the government was working closely with Internet security agencies and others to deal with the problem. It urged computer users to download anti-virus programmes.

A DDoS attack often uses viruses planted in "zombie" computers. These seek simultaneous access to selected sites and swamp them with traffic.

In July 2009 a major cyber-attack temporarily shut down 25 sites domestically and in the United States, including the State Department, the White House and the Pentagon.

South Korea's spy chief reportedly blamed North Korea's telecommunications ministry for that incident, although US officials reached no conclusion.

In July last year, on the anniversary of the first incident, a number of websites suffered similar DDoS attacks because some contaminated PCs had not been fixed. (AFP)