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A number of official North Korean websites, including those of the Korean Central News Agency (KCNA), the daily Rodong Sinmun newspaper, and Air Koryo airline became inaccessible early Wednesday.
"Internet servers operated by our republic have come under daily cyber attack(s) which are persistent and intensive", said KCNA, which noted that the problem coincided with an ongoing South Korea-US joint military drill.
Accusing the United States and its South Korean "puppets" of building up their cyber-war capabilities, KCNA said the attack was a "cowardly and despicable act" motivated by fear.
Military tensions on the Korean peninsula have escalated dramatically since the North conducted its third nuclear test last month.
Pyongyang responded to the subsequent UN sanctions -- and joint military exercise -- with threats of "all-out war" backed by nuclear weapons.
"We'll never sit idle in the face of such cyber attacks by the enemy... which have reached an extremely reckless and grave stage," KCNA said.
There was no immediate response from the South Korean government to the accusations of involvement.
In a report datelined Pyongyang, Russia's ITAR-TASS news agency cited an informed source blaming "a powerful hacker from abroad" for the attack.
Most of the North Korean websites were back up by late Thursday.
While North Koreans live in probably the most isolated and censored society on the planet and one that comes near the bottom of any media freedom survey, the country is not a complete IT desert.
North Korea launched a domestic intranet in 2008, which is cut off from the rest of the world, allowing its very limited number of users to exchange state-approved information and little more.
Access to the full-blown Internet is for the super-elite only, meaning a few hundred people or maybe 1,000 at most, analysts estimate.
Charges of state-sanctioned hacking have usually flowed in the opposite direction.
South Korea accused the North of being behind large-scale cyber attacks on the websites of its government agencies and financial institutions in July 2009 and March 2011.
Seoul also denounced North Korea for jamming the GPS systems of hundreds of civilian aircraft and ships in South Korea in April and May last year.