North Korea, at the centre of a confrontation with the US over the hacking of Sony Pictures, experienced a complete Internet outage for hours before links were restored.
US-based Dyn, a company that monitors Internet infrastructure, said the reason for the outage was not known, but could range from technological glitches to a hacking attack.
Several US officials insisted the US government had not taken any cyber action against Pyongyang.
US President Barack Obama had vowed on Friday to respond to the major cyberattack, which he blamed on North Korea, "in a place and time and manner that we choose".
Dyn said North Korea's Internet links were unstable yesterday and the country later went completely offline.
The possible causes could be attacks by individuals, a hardware failure, or even North Korea itself, experts said.
Matthew Prince, CEO of US-based CloudFlare which protects websites from web-based attacks, said the fact that North Korea's Internet was back up "is pretty good evidence that it wasn't a state-sponsored attack".
North Korea has denied it was behind the Sony cyberattack and vowed to hit back against any US retaliation.
The hackers said they were incensed by a Sony comedy, The Interview, about a fictional assassination of North Korean leader Kim Jong Un, which the movie studio has now pulled from general release.