New Hampshire-based Dyn Research said the reason for the disruptions 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 cyberaction against Pyongyang.
Washington last Thursday requested China's help, asking Beijing 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 Korea 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 cyber-attacks and that there was no proof that North Korea was responsible for the Sony hacking.
Trouble staying online
North Korea has denied it was behind the cyber-attack and has vowed to hit back against any U.S. retaliation.
The United States had also asked several other countries in the region for help, including Japan, which strongly condemned the attacks but stopped short of blaming North Korea.
Doug Madory, director of Internet analysis at Dyn Research, which detected the North Korean Internet outage, said: "For the past 24 hours North Korea's connectivity to the outside world has been progressively getting degraded to the point now that they are totally offline.
"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," he said.
Current and former U.S. law enforcement and security officials said only a tiny number of people in North Korea's leadership have access to the Internet, and that almost all its Internet links and traffic pass through China.
In its statement on Monday, China made no reference to calls by the United States for joint action with it and other countries to counter any similar cyber-attacks.
"Before making any conclusions there has to be a full (accounting of) the facts and foundation," Foreign Ministry spokeswoman Hua Chunying said. "China will handle it in accordance with relevant international and Chinese laws according to the facts."
She said Chinese Foreign Minister Wang Yi, in a conversation with U.S. Secretary of State John Kerry on Sunday, "reaffirmed China's relevant position, emphasizing China opposes all forms of cyber-attacks and cyberterrorism."
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 cyberspying in the past and a U.S. official has said the attack on Sony could have used Chinese servers to mask its origin.
South Korea, which is still technically at war with North Korea, said computer systems at its nuclear plant operator had been hacked and non-critical data stolen but that there was no risk to nuclear installations or reactors.
"It's our judgment that the control system itself is designed in such a way there is no risk whatsoever," Chung Yang-ho, deputy energy minister, said by telephone.
He made no mention of North Korea and could not verify messages posted by a Twitter user claiming responsibility for the attacks and demanding the shutdown of three ageing nuclear reactors by Thursday.
North Korea has denied it was to blame for the attack and has vowed to hit back against U.S. retaliation, threatening the White House and the Pentagon.
At the United Nations, U.S. Ambassador Samantha Power dismissed as absurd Pyongyang's demand for a joint U.S.-North Korean investigation of the hacking of Sony Pictures and threats of retaliation if the United States refused. "It is exactly the kind of behavior we have come to expect from a regime that threatened to take 'merciless countermeasures' against the U.S. over a Hollywood comedy, and has no qualms about holding tens of thousands of people in harrowing gulags," Power said.
Several U.S. officials said that the FBI, which has taken the lead inside the government in investigating the Sony hack, had help from other agencies, including spy agencies, among them the secretive National Security Agency.
The NSA's cyber-security and cyberspying capabilities are the most formidable of any U.S. agency, but it has been under criticism since former contractor Edward Snowden disclosed secrets of the agency's surveillance mechanisms last year.
© Thomson Reuters 2014