South Korea has issued a cyber attack alert after a number of its official and private websites were targeted on the sensitive anniversary of the start of the Korean War.
Seoul's science ministry said unidentified hackers attacked several government websites, including that of the presidential Blue House, and some news media servers on Tuesday.
The statement said a five-stage national cyber attack alert has been raised from level one to level two, warning officials and citizens to take extra computer security measures.
Tuesday is the 63rd anniversary of the start of the Korean War. But it is unclear whether the attacks were related to the anniversary, and there were conflicting signs about who may be responsible.
Some of the hacked South Korean websites on Tuesday temporarily posted messages in support of North Korean leader Kim Jong Un. A Twitter account said to be run by a member of Anonymous, a loosely associated global network of hacking activists, also claimed some of the attacks.
But other purported members of Anonymous also used Twitter to claim attacks on several websites in North Korea, including the North's official Korean Central News Agency and Rodung Sinmun news outlets. Both websites experienced outages Tuesday.
The two Koreas have accused each other of cyber attacks in recent months.
South Korea blamed North Korea's military spy agency for a March cyber attack that affected 48,000 computers and servers, stalling operations at three top South Korean broadcasters and hampering financial services at banks for several days.
The South's Korea Internet and Security Agency (KISA) found the attack bore similarities to previous hacking attempts by Pyongyang's military-run Reconnaissance General Bureau.
North Korea is believed to have an elite cyber warfare unit that was suspected of being behind computer attacks on South Korean government agencies and financial institutions in 2009 and 2011.
Pyongyang, meanwhile, accused South Korea and the United States of bringing down several of its websites in a March attack.