South Korea has issued a cyber attack alert after hackers penetrated a number of official websites, including the presidential "Blue House" website, on the anniversary of the start of the Korean War.

"The government can confirm a cyber attack by unidentified hackers that shut down several sites including the Blue House," the science ministry said in a statement released on Tuesday, adding that the five-stage national cyber alert had been raised from level one to two.

A number of news media websites and several government agencies, including the office for Government Policy Coordination and the ruling New Frontier Party, were also affected by what seemed to be a coordinated attack beginning at 10:45am local time (01:45 GMT).

War anniversary

The hacking coincided with the 63rd anniversary of the start of the Korean War on June 25, 1950. The two countries are technically still at war, as the conflict ended with an armistice rather than a peace treaty.

The ministry statement did not speculate on who might be responsible. Investigations into several recent large-scale cyber assaults on South Korean media groups and financial institutions concluded that they originated in North Korea.

A number of posts left on the hacked sites claimed to be the work of the global "hacktivist" group Anonymous and included messages praising North Korean leader Kim Jong-Un.

Anonymous denied any involvement on its official Twitter account, but said it had succeeded in hacking a number of North Korean media websites on Tuesday, including the official Korean Central News Agency (KCNA) and the ruling party newspaper, Rodong Sinmun. Both sites were briefly inaccessible on Tuesday morning, but appeared to be running normally a few hours later.

As of 07:00 GMT, the Blue House website was back online after having been shut down with a message saying the site was "under maintenance" for hours earlier.

South Korea has sought to beef up its cyber defences since a March 20 attack completely shut down the networks of TV broadcasters KBS, MBC and YTN, and halted financial services and crippled operations at three banks.

An official investigation determined North Korea's military intelligence agency was responsible, with a joint team of civilian and government experts tracing the origin to six personal computers used in North Korea.