Lee Jin-man/AP Photo

Major websites in both North and South Korea are under a series of cyberattacks after international hacking group Anonymous warned last week it planned to disclose North Korea's military documents today.

The targeted state-run North Korean websites Rodong Sinmun, Korea Central News Agency and Air Koryo, among others, were down or temporarily blocked today. Twitter accounts of Anonymous were tweeting live as the attacks were underway.

One Korean Twitter account suspected as that of hacker @Anonsj tweeted, "is #TANGODOWN ~ " just minutes after the Pyongyang-run website went blank.

Anonymous had widely advertised its plans to infiltrate North Korean websites through a video message posted on YouTube June 17.

"Your major missile documentation and residents, military documents show down is already in progress … We are partially sharing this information with the world," the two-minute video clip said.

The group claimed it will "no longer tolerate North Korea's way of ruling and will work towards world peace."

Anonymous had warned that the cyberattack would be conducted today, the 63 rd anniversary of the beginning of the Korean War in 1950, to prove its strength.

North Korea has accused the U.S. and South Korea governments of backing Anonymous but released no immediate statement today.

In South Korea, dozens of websites were also temporarily disabled throughout the day, including the official site for the presidential office, major government agencies and media companies such as Chosun Ilbo.

The Presidential Office website went down for 10 minutes from 10 a.m. local time with Anonymous' signature black skull and masks displayed on screen with the message "hacked by Anonymous" and "Long live the Unification President, Great Leader Kim Jong-un."

The global Anonymous denied it was behind the attacks on South Korean websites and insisted North Korean hackers were framing the group in retaliation by staging the attack on South Korean websites to make it look like Anonymous was responsible.

The damage was seen as little threat to national security, but the South Korean government issued a cyberattack alert, raising the five-stage national cyberalert from level one to two.

"The government can confirm a cyberattack by unidentified hackers that shut down several sites, including the Blue House," the science ministry said in a statement.

It did not speculate who might be responsible.

ABC News' Joanne Kim contributed to this report.