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, according to a US company that monitors internet infrastructure.
Dyn, the New Hampshire-based internet monitor, said on Tuesday the reason for the outage was not known but could range from technological glitches to a hacking attack.
Several US officials close to the investigations of the attack on Sony Pictures said the US government was not involved in any cyber action against North Korea.
US President Barack Obama had pledged 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 on Monday, and the country later went completely offline.
"We're yet to see how stable the new connection is," Jim Cowie, chief scientist for the company, said in a telephone call to Reuters news agency after the services were restored.
"The question for the next few hours is whether it will return to the unstable fluctuations we saw before the outage."
North Korea is one of the most isolated nations in the world, and the effects of the internet outage there were not fully clear.
Very few of North Korea's 24 million people have access to the internet.
However, major websites, including those of the KCNA state news agency, the main Rodong Sinmun newspaper and the main external public-relations company went down for hours.
Almost all of the country's internet links and traffic pass through China, except, possibly, for some satellite links.
"North Korea has significantly less internet to lose, compared to other countries with similar populations: Yemen [47 networks], Afghanistan [370 networks], or Taiwan [5,030 networks]," Dyn Research said in a report.
"And unlike these countries, North Korea maintains dependence on a single international provider, China Unicom."
Meanwhile, South Korea, which remains technically at war with the North, said it could not rule out the involvement of its neighbour in a cyberattack on its nuclear power plant operator.
It said only non-critical data was stolen and operations were not at risk, but had asked for US help in investigating.
Park Geun-hye, South Korean president, said on Tuesday the leak of data from the nuclear operator was a "grave situation" that was unacceptable as a matter of national security, but she did not mention any involvement of North Korea.