It comes days after President Obama pledged a "proportional response" to the communist country's alleged hacking of Sony Pictures. It's unclear what caused the outage.
North Korea's National Defense Commission, which is headed by the country's leader, Kim Jong Un, said its military was ready to fight America "in all war spaces including cyber warfare space."
On CNN's State of the Union, the president expanded on earlier remarks he made criticizing a decision by Sony Pictures to pull distribution of The Interview.
"That's not what America's about," the president said in a news conference, calling the cancellation of The Interview's release a "mistake." The FBI has accused North Korea of being behind the attack.
The recent attack on Sony Pictures' computer network that resulted in a flood of confidential data has its origins in North Korea, U.S. intelligence officials say.
The cyberattack successfully achieved its unusually public goals. The question, reporter David Sanger says, is how the United States can punish what already is the world's most-sanctioned nation.
A North Korean official now denies its involvement in one of the worst corporate hacks in history, after a different official played coy. How sophisticated are the Hermit Kingdom's hackers?
A cyber attack on Sony may have been done by North Koreans in response to an new comedy about an attempt to kill Kim Jong Un. Huge amounts of personal data and five films have been leaked so far.