Major government and media websites in South and North Korea have been shut down for hours on the 63rd anniversary of the start of the Korean War.
Seoul said its sites were hacked on Tuesday, while it was unclear what knocked out those north of the border.
Seoul said experts were investigating attacks on the websites of the South Korean presidential Blue House and prime minister's office, as well as some media servers. There were no initial reports that sensitive military or other key infrastructure had been compromised.
The attacks in South Korea did not appear to be as serious as a cyberattack in March which shut down tens of thousands of computers and servers at South Korean broadcasters and banks. Seoul warned people to take security measures against cyberattacks.
North Korean websites which were shut down on Tuesday included those belonging to the national airline, Air Koryo, the Rodong Sinmun newspaper, the North's official Uriminzokkiri site and Naenara, the country's state-run internet portal. All but Air Koryo were operational a few hours later.
South Korean National Intelligence Service officials said they were investigating what may have caused the shutdown of the North Korean websites. North Korea did not make any immediate comment.
Operators of several Twitter accounts who purported to be part of a global hackers' collective known as Anonymous claimed that they had attacked North Korean websites. The Associated Press received no answer to several requests to speak to the Twitter users.
Shin Hong-soon, an official at South Korea's science ministry in charge of online security, said the government was not able to confirm whether these hackers were linked to today's attack on South Korean websites.
North and South Korea have traded accusations of cyberattacks in recent years.
The shutdowns came on a war anniversary that both countries were marking with commemorations. They also are gearing up for the 60th anniversary of the end of the fighting on July 27, a day North Koreans call "Victory Day" even though the Korean War ended in a truce, not a peace treaty.