Updated 5:55 a.m. ET
SEOUL, South Korea Major government and media websites in South and North Korea were shut down for hours Tuesday on the 63rd anniversary of the start of the Korean War. Seoul said its sites were hacked, 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 Tuesday that sensitive military or other key infrastructure had been compromised.
The attacks in South Korea did not appear to be as serious as a March cyberattack that shut down tens of thousands of computers and servers at South Korean broadcasters and banks. Seoul alerted people to take security measures against cyberattacks.
The North Korean websites that shut down 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 didn't 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 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 Tuesday's attack on South Korean websites.
It wasn't immediately clear who was responsible. North and South Korea have traded accusations of cyberattacks in recent years.
South Korean officials blamed Pyongyang for a March 20 cyberattack that struck 48,000 computers and servers, hampering banks and broadcasters for several days, although television programming was not interrupted and officials have said that no bank records or personal data were compromised. Seoul officials said in April that an initial investigation pointed to a North Korean military-run spy agency as the culprit.
North Korea blamed South Korea and the United States for cyberattacks in March that temporarily disabled Internet access and websites in North Korea.
Experts believe North Korea trains large teams of cyber warriors and that the South and its allies should be prepared against possible attacks on key infrastructure and military systems. If the inter-Korean conflict were to move into cyberspace, South Korea's deeply wired society would have more to lose than North Korea's, which remains largely offline.
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 July 27, a day North Koreans call "Victory Day" even though the Korean War ended in a truce, not a peace treaty.
Tens of thousands of North Koreans were gathering Tuesday to Pyongyang's main Kim Il Sung Square for the largest of many rallies around the nation denouncing the United States. On Monday evening, men lined up in the shadow of the capital's iconic Juche Tower to practice coordinating their steps as they hoisted signs reading "Sweep away the imperialist American aggressors," "sworn enemies," and "U.S. troops out of South Korea" while a man with a megaphone barked out orders.
In South Korea, thousands of people, including Korean war veterans, gathered at Jamsil Stadium in Seoul for a commemoration. Two South Korean army units held military drills in Yeoncheon in Gyeonggi Province, near the demilitarized zone, defense officials said in Seoul.
North Korea in recent weeks has pushed for diplomatic talks with Washington. Tensions ran high on the Korean Peninsula in March and April, with North Korea delivering regular threats over U.N. sanctions and U.S.-South Korean military drills.