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 (0145 GMT).
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.