South Korea issued a cyber security alert as the websites of 29 government and other agencies came under attack, the Korea Communications Commission said.

A commission spokesman said the DDoS (distributed denial of service) attacks had initially been expected to hit 40 websites but only 29 were actually affected.

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

"Some of the sites suffered minor access problems," he said.

AhnLab, a local IT security company, said its own website had also come under attack along with those of seven major banks in the incident that began at 10:00 am (0100 GMT).

The commission 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 those of 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.