North Korea is experiencing serious internet problems, with experts saying that online access is "totally down".
It is not immediately clear if the internet connectivity problems were an act of retribution for a major intrusion at Sony Pictures Entertainment which the FBI last week linked to North Korea.
US president Barack Obama on Friday said the US government would respond but did not say how.
The White House declined to comment Monday on the problems but State Department spokeswoman Marie Harf said of the federal government responses that "some will be seen, some may not be seen".
Doug Madory, director of internet analysis at Dyn Research, said the internet connectivity problems were discovered in the last 24 hours and have gotten progressively worse to the point that "North Korea's totally down".
North Korea had earlier threatened strikes against the White House, Pentagon and "the whole US mainland, that cesspool of terrorism", accusing Mr Obama of "recklessly" spreading rumours of a Pyongyang-orchestrated cyber-attack on Sony Pictures.
The US blames North Korea for the cyber-attack that escalated to threats of terror strikes against American cinemas and caused Sony to cancel The Interview’s release.
Mr Obama, who promised to respond "proportionately" to the attack, has said Washington is reviewing whether to put North Korea back on its list of state sponsors of terrorism.
The National Defence Commission, led by Mr Kim, warned that its 1.2 million-member army was ready to use all types of warfare against the US.
“Our toughest counter-action will be boldly taken against the White House, the Pentagon and the whole US mainland, the cesspool of terrorism, by far surpassing the ‘symmetric counter-action’ declared by Obama,” said the commission’s policy department, in a statement carried by the official Korean Central News Agency.
North Korea has said it knows how to prove it had nothing to do with the hacking and proposed a joint investigation with the US.
The rivals are locked in an international stand-off over the North’s nuclear and missile programmes and its alleged human rights abuses. Last spring tension dramatically rose after North Korea issued a string of fiery threats to launch nuclear strikes against Washington and Seoul.
AP