Cyberattacks Shut Down Leading Korean Sites SEOUL, South Korea - Major government and news media Web sites in South and North Korea were shut down on Tuesday after anonymous hackers claimed to have attacked them on the 63rd anniversary of the start of the 1950-53 Korean War. It remained unclear who the hackers were and whether the attacks on North and South Korea came from the same sources. With the Web sites of its presidential office and at least one other government agency down, the South Korean government issued an alert on Tuesday warning the people against cyberattacks. The Web sites belonging to North Korean institutions including Air Koryo, the main party newspaper Rodong Sinmun, the news agency Korean Central News Agency and several other entities were inaccessible on Tuesday. Neither North Korea nor South Korean officials commented on what may have caused the Internet disruptions in the isolated North, where only a handful of people use the Internet and all Web sites are tightly controlled by the state. But people who have claimed themselves to be a global network of hackers called "Anonymous" have warned through Twitter that they would attack North Korean Web sites on Tuesday. Among other things, they criticized North Korea for keeping most of its people off the Internet. South Korean officials said they were investigating who was behind the hacking attacks on their Web sites. According to the national news agency Yonhap, when the attack began on Tuesday, the Web site of the presidential Blue House was splashed with a large hacker's message that said in Korean: "Hurrahs to Kim Jong-un, the president of a unified Korea!" Mr. Kim is the leader of North Korea. Yonhap also cited Twitter users who claimed responsibility for the attacks in South Korea as demanding that the Seoul government stop censoring Internet content and that its intelligence agency apologize for a recent political scandal in which government intelligence agents were accused of engaging in an online campaign to attack opposition candidates ahead of the Dec. 19 presidential election. The two Koreas have accused each other of launching hacking attacks at their respective Web sites in recent years. In March, a series of cyberattacks paralyzed the computer networks of three broadcasters and three banks, as well as several government Web sites in South Korea. South Korea officially blamed North Korea in April for the attacks. South Korea is one of the world's most wired societies. Calls have increased in recent years for better protection against cyberattacks from the North, which South Korean officials said have trained an army of cyberwarriors. Before the March attacks, South Korea's National Intelligence Service had suspected North Korea's involvement in at least six earlier hacking attacks reported in South Korea since 2008, including the huge disruptions that crashed South Korean government Web sites in 2009 and 2011. Meanwhile, North Korea blamed South Korea and the United States for cyberattacks in March that temporarily disabled Internet access and Web sites in North Korea.