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, but U.S. officials said Washington was not involved.
U.S.-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 U.S. officials close to the investigations of the attack on Sony Pictures said the U.S. government had not taken any cyber action against Pyongyang.
U.S. 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 on Monday and the country later went completely offline. Links were restored at 0146 GMT on Tuesday, and the possibilities for the outage could be attacks by individuals, a hardware failure, or even that it was done by North Korea itself, experts said.
Matthew Prince, CEO of U.S.-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 the outage wasn't caused by a state-sponsored attack, otherwise it'd likely still be down for the count".
Almost all of North Korea's Internet links and traffic pass through China and it dismissed any suggestion that it was involved as "irresponsible".
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 least-connected nations in the world, and the effects of the Internet outage would have been minimal.
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.
"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.
The Chinese Foreign Ministry said on Monday it opposed all forms of cyberattacks but 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.
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 and Sohee Kim in Seoul; David Brunnstrom and Mark Hosenball in Washington; Ben Blanchard and Megha Rajagopalan in Beijing; Jeremy Wagstaff in Singapore; Editing by Raju Gopalakrishnan)
