SEOUL (AFP) - South Korea issued a cyber attack alert on Tuesday after hackers apparently penetrated a number of official websites, including the presidential Blue House, the science ministry said.

"The government can confirm a cyber attack by unidentified hackers that shut down several sites including the Blue House," the ministry said in a statement, 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 were also affected by what seemed to be a coordinated attack beginning at 10.45 am (9.45am Singapore time).

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.