By Jack Kim and Lesley Wroughton
SEOUL/WASHINGTON (Reuters) - North Korea, at the center of a confrontation with the United States over the hacking of Sony Pictures, experienced a complete Internet outage for hours before links were restored on Tuesday, a U.S. company that monitors Internet infrastructure said.
New Hampshire-based Dyn said the reason for the outage was not known but could range from technological glitches to a hacking attack. Several U.S. officials close to the investigations of the attack on Sony Pictures said the U.S. government was not involved in any cyber action against Pyongyang.
U.S. President Barack Obama had vowed on Friday to respond to the major cyber attack, 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 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."
Meanwhile South Korea, which remains technically at war with the North, said it could not rule out the involvement of its isolated neighbor 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 U.S. help in investigating.
South Korean President Park Geun-hye 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.
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 its 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 its 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."
NO PROOF, CHINA SAYS
The United States requested China's help last Thursday, asking it to shut down servers and routers used by North Korea that run through Chinese networks, senior administration officials told Reuters.
The United States also asked China to identify any North Korean hackers operating in China and, if found, send them back to North Korea. It wants China to send a strong message to Pyongyang that such acts will not be tolerated, the officials said.
By Monday, China had not responded directly to the U.S. requests, the officials added.
In Beijing, the Chinese Foreign Ministry said on Monday it opposed all forms of cyberattacks and that there was no proof that North Korea was responsible for the Sony hacking.
North Korea has denied it was behind the cyberattack on Sony and has vowed to hit back against any U.S. retaliation, threatening the White House and the Pentagon..
The hackers said they were incensed by a Sony comedy about a fictional assassination of North Korean leader Kim Jong Un, which the movie studio has now pulled from general release.
Doug Madory, director of Internet analysis at Dyn Research, said of the outage in North Korea:
"There's either a benign explanation - their routers are perhaps having a software glitch; that’s possible. It also seems possible that somebody can be directing some sort of an attack against them and they're having trouble staying online."
Other experts said it was possible North Korea was attacked by hackers using a botnet, a cluster of infected computers controlled remotely.
"It would be possible that a patriotic actor could achieve the same results with a botnet, however the President promised a proportional response," said Tom Kellermann, Chief Cybersecurity Officer at Trend Micro.
"The real issue here is that nonstate actors and rogue regimes will adopt this modus operandi in 2015. The use of destructive cyberattacks will become mainstream."
China is North Korea's only major ally and would be central to any U.S. efforts to crack down on the isolated state. But the United States has also accused China of cyber spying in the past and a U.S. official has said the attack on Sony could have used Chinese servers to mask its origin.
(Additional reporting by Meeyoung Cho in Seoul; David Brunnstrom and Mark Hosenball in Washington; Ben Blanchard and Megha Rajagopalan in Beijing; Jeremy Wagstaff in Singapore; Editing by Raju Gopalakrishnan)