North Korea is experiencing widespread Internet outages. One expert says the country's online access is “totally down.”
It's not immediately clear if the Internet connectivity problems were an act of retribution for a major intrusion at Sony Pictures Entertainment that the FBI last week linked to North Korea.
President Barack Obama on Friday said the US government would respond but didn't say how.
The White House declined to comment Monday. State Department spokeswoman Marie Harf told reporters that of the federal government responses, "some will be seen, some may not be seen."
Sony Cyber Attack: The Worst Affected In Hollywood
Sony Cyber Attack: The Worst Affected In Hollywood
1/13 Leonardo DiCaprio
The actor's 'horrible behaviour' over Steve Jobs biopic was branded 'actually despicable' in leaked Sony executive emails
2/13 Seth Rogen
Seth Rogen and James Franco cancel Interview (North Korea-baiting film) press appearances after Sony hackers' threats
3/13 James Franco
Seth Rogen and James Franco cancel Interview press appearances after Sony hackers' threats
4/13 Aaron Sorkin
The screenwriter has said that the people who hacked into Sony Entertainment Pictures and released private emails are 'threatening the lives of whole families'
Theo Wargo | Getty Images for Nantucket Film Festival
5/13 Amy Pascal
Also leaked: Sony Pictures co-chair Amy Pascal's email joking about President Barack Obama's race
6/13 George Clooney
Leaked email chain between Pascal and George Clooney appeared to reveal just how much the actor struggled to cope with the criticism of his The Monuments Men
7/13 Barack Obama
Sony executives Scott Rudin and Amy Pascal's leaked email exchange jokes about Barack Obama favouring films with black actors
8/13 Channing Tatum
Channing Tatum's leaked Sony executive email responding to the success of 22 Jump Street with Jonah Hill, which became the highest grossing R-rated comedy ever when it opened in June this year
9/13 Jaden and Willow Smith
Will Smith and Jada Pinkett Smith's children Willow and Jaden Smith found themselves in the Sony inbox as well
10/13 Mark Gordon
DiCaprio's exchange with Jobs TV producer Mark Gordon (pictured) was leaked too
11/13 Princess Beatrice
Sony hackers linked to North Korea revealed Princess Beatrice earns $30,000 a year
12/13 Michael Fassbender
Aaron Sorkin's leaked emails claimed he had no idea who actor Michael Fassbender (pictured) was
13/13 Scott Rudin
Information leaked has also included embarrassing emails involving high profile producer Scott Rudin attacking Angelina Jolie
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, told CNN's State of the Union that Washington as reviewing whether to put North Korea back on its list of state sponsors of terrorism.
The National Defence Commission, led by 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.
Sony Pictures Entertainment, which said it cancelled the cinema release of The Interview after distributors refused to show it, pledged to find a way to get the film out.
"How it's going to be distributed, I don't think anybody knows quite yet," a Sony lawyer said.
Mr Obama said of North Korea: "We're not going to be intimidated by some cyber-hackers." The president said the US would examine the facts to determine whether North Korea should go back on the terrorism sponsors list.
"We're going to review those through a process that's already in place," he said. "I'll wait to review what the findings are."
But while raising the possibility of a terrorism designation, the president also asserted: "I don't think it was an act of war. I think it was an act of cyber-vandalism that was very costly, very expensive. We take it very seriously."
That description drew immediate scorn from two Republicans and long-time critics of Mr Obama's foreign policy.
"It is a new form of warfare and we have to counter with that form of warfare with a better form of warfare," said Arizona senator John McCain.
Senator Lindsey Graham of South Carolina called it "an act of terrorism" and favoured reimposing sanctions and adding North Korea to the terrorism list. The US needed to "make is so hard on the North Koreans that they don't want to do this in the future", he said.
The US is asking China for help as it considers how to respond to the hack. A senior Obama administration official says America and China have shared information about the attack and Washington has asked for Beijing's co-operation.
China wields considerable leverage over North Korea, but Mr Obama has also accused the country of carrying out cyber-thefts.
Today North Korea refused to turn up to what would have been a groundbreaking discussion of its human rights situation at the UN. Members of the UN Security Council voted to override China's objections and formally add the issue to the council's agenda.Reuse content