The Anonymous hacktivist group said it attacked North Korea's main news and propaganda Web site, Uriminzokkiri, on Kim Il-sung's birthday. The birthday that fell on Monday is called the Day of the Sun and is the most important holiday in the North. Kim who died in 1994 is the grandfather of incumbent leader Kim Jong-un.
The attacks come after the hackers claimed earlier in the month that they "crashed" several North Korean sites to protest Pyongyang's efforts to build nuclear weapons. The North detonated its third nuclear device on Feb. 12, in defiance of warnings by the global community.
The loosely associated organization of hackers from many countries also said they have defaced the Minjok Tongshin, an English language news provider, Paekdu-Hanna.com, Jajusasang.com and Ryugyong Clip, a video and still image site, in their latest attacks.
A local hacker, who is a member of Anonymous's South Korean chapter, claimed on a Twitter account that the collapse of North Korea's network system is near, although he gave no details for making such an assertion.
The group, meanwhile, caused a sensation when it attacked and defaced Uriminzokkiri and stole 15,000 personal data of registered members that were subsequently released for public viewing.
South Korean law enforcement authorities have launched a probe to see if South Koreans who registered with the propaganda site violated related domestic laws that can punish people for helping the North.
The two Koreas remain technically at war since the Korean War (1950-53) ended in an armistice, not a peace treaty.