Official sites such as North Korea's Korean Central News Agency, Air Koryo and the party newspaper Rodong Sinmun have been inaccessible periodically, the BBC reported Friday.
Despite the periodic disruptions, the websites for the state news agency and party newspaper were accessible Friday, The New York Times reported.
"It is nobody's secret that the U.S. and South Korean puppet regime are massively bolstering up cyberforces in a bid to intensify the subversive activities and sabotages against [the Democratic People's Republic of Korea]," the North's Korean Central News Agency said. "They are seriously mistaken if they think they can quell the DPRK's voices of justice through such base acts."
North Korea has been accused of hacking into government websites and disrupting Internet service to South Korea.
North Korea did not explained the "intensive and persistent virus attacks," but Russia's ITAR-Tass news agency reported a "powerful hacker attack" from abroad crashed North Korea's Internet servers, the Times said.
Internet access in North Korea is limited, with most people able to access only a few of state-run pages, the BBC said. Broader Internet is available to the government and the military.
North Korea's rhetoric has escalated in recent weeks, and included a threat to launch pre-emptive nuclear strikes against the United States and South Korea after the allies began joint military drills, followed by new U.N. sanctions for North Korea's Feb. 12 nuclear test. North Korea also said it voided the Korean conflict cease-fire and non-aggression agreements with South Korea, as well as cut a hotline between the two countries.