Widespread internet outage in North Korea has been blamed on a cyber-attack by the US, in retaliation for the hacking that intimidated Sony into cancelling a movie release and left the country feeling humiliated and angry.
The White House and the State Department declined to say whether the US government was responsible, after North Korea experienced sweeping internet outages for hours before coming back online late on Monday.
The US had promised a "proportional response" to the Sony hacking.
One computer expert said the country's online access was "totally down."
A poster in South Korea shows how cyber war can be waged against the North
After the Sony hacking led to release of comedy 'The Interview' being cancelled, Barack Obama said the US government expected to respond to it.
He described it as an expensive act of "cyber vandalism", which led to the leaking of private emails, and an FBI investigation blamed North Korea.
He did not say how the US might respond and its offensive cyber operations as highly classified.
"We aren't going to discuss, publicly operational details about the possible response options or comment on those kind of reports in anyway except to say that as we implement our responses, some will be seen, some may not be seen," State Department spokeswoman Marie Harf said.
North Korea has forcefully denied it was responsible for hacking into Sony. But the country has for months condemned the 'The Interview', a comedy about a plot to assassinate the North Korean leader Kim Jong-Un.
Sony cancelled plans to release the movie after a group of hackers made terroristic threats against cinemas that planned to show it.
In North Korea, one of the most isolated states in the world, few have access to computers, and even those who do are typically able to connect only to a domestic intranet.
Though the country is equipped for broadband Internet, only a small, approved segment of the population has any access to the World Wide Web.
More than a million people, however, are now using mobile phones in North Korea. The network covers most major cities but users cannot call outside the country or receive calls from outside.
Ivan Simonovic, the UN assistant secretary-general for human rights, said he couldn't speculate about the origin of the internet outages but he hoped it would be "thoroughly investigated."
Doug Madory, the director of Internet analysis at Dyn Research, an Internet performance company, said the problems began over the weekend and grew progressively worse to the point that "North Korea's totally down."
Another Internet technology service, Arbor Networks, which protects companies against hacker attacks, said its monitoring detected denial-of-service attacks aimed at North Korea's infrastructure starting on Saturday and persisting into Monday.
But North Korea's limited connectivity and lack of Internet sophistication would make it relatively simple for a band of hacktivists to shut down online access, and we should not assume that the US government had any part, said Dan Holden, director of security research at Arbor Networks.
"Anyone of us that was upset because we couldn't watch the movie, you could do that. Their Internet is just not that sophisticated," he said.