Pyongyang’s official news agency KCNA said the cyber attacks coincided with the ongoing U.S.-South Korea joint military drills.
Official sites of KCNA, Air Koryo and Rodong Sinmun, the party newspaper, were reported to have been inaccessible on some occasions in recent days.
Russia's Itar-Tass news agency said a "powerful hacker attack" from abroad had brought down Internet servers inside the North, deeming some websites inaccessible.
The 11-day U.S.-South Korean joint military drills that started Monday involve 10,000 South Korean and about 3,000 U.S. troops. Those coincide with two months of separate U.S.-South Korean field exercises that began March 1, according to an Associated Press report.
KCNA said it was "nobody's secret that the U.S. and South Korean puppet regime are massively bolstering up cyber forces in a bid to intensify the subversive activities and sabotages against the DPRK [North Korea].”
“Intensive and persistent virus attacks are being made every day on internet servers operated by the DPRK,” the KCNA commentary said, describing the attacks as “despicable” and “ridiculous.”
Last week, North Korea had threatened to launch a pre-emptive nuclear attack against the U.S.
Earlier this week, Pyongyang turned up the heat, declaring that it was scrapping the armistice that ended the Korean War 60 years ago. It has also cut off a hotline that connects the two countries. However, South Korea said a unilateral move to end the pact was not legally possible.
The two Koreas are still technically considered to be at war since the 1950-53 Korean War ended with an armistice pact, not a peace treaty.
The marked escalation in war rhetoric from Pyongyang followed international condemnation of its nuclear test in February.
Pyongyang opposes the fresh U.N. sanctions following the test and has also voiced its disapproval of the two annual joint U.S.-South Korea military exercises.
North Korea's third nuclear test followed an apparently successful launch in December of a three-stage rocket, which drew international criticism for testing of missile technology banned under a U.N. Security Council resolution.