North Korea has been knocked offline, news reports say.

The outages to the secretive nation’s four official Internet networks began Sunday and as of Monday all were totally offline, Bloomberg reported.

The outage was initially reported by Dyn, a company in Manchester, N.H., that tracks Internet traffic and performance.

The company’s researchers tweeted that “After 24 hours of increasing instability, North Korean national Internet has been down hard for more than 2 hours.”

The company posted a chart showing the outage.

North Korea’s Internet access is routed through China. However, who is behind the outages is unknown.

U.S. officials on Monday declined to say if the United States was responsible for the outage.

On Friday, President Obama said he would “respond proportionately” to the cyberattack on Sony Pictures Entertainment, which the FBI confirmed was launched by North Korea.

However he was very clear Sunday that the hacking was not an act of war. Speaking on CNN’s State of the Union, he said, “I think it was an act of cyber vandalism that was very costly, very expensive. We take it very seriously. We will respond proportionately.”

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On Sunday North Korea’s National Defense Commission threatened military strikes against the United States, “the ill-famed cesspool of injustice” in its words, in response to the accusation that it was behind the hack attack.

In a statement, it said, “The army and people of the DPRK (Democratic People’s Republic of Korea) are fully ready to stand in confrontation with the U.S. in all war spaces including cyber warfare space to blow up those citadels.”

North Korea has consistently denied involvement in the attack, which included threats made to theaters and moviegoers who went to see the Sony film The Interview, a CIA spoof centered around a plot to assassinate North Korean leader Kim Jong Un.

Sony pulled the movie last week, but Sony lawyers said on Sunday that it would be released, perhaps as a free streaming movie.

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Contributing: The Associated Press