North Korea's Internet went completely dark Monday after hours of instability, cyber experts said, suggesting the country's network could be under attack following the hacking of Sony Pictures.

"North Korea is completely off the Internet," Earl Zmijewski, vice president of data analytics at respected cyber security firm Dyn Research, told AFP.

Pyongyang's connection woes came after US President Barack Obama vowed to retaliate for what the FBI said was North Korea's cyber assault on the Hollywood studio.

While US officials refused to confirm whether Washington has already taken action against Pyongyang, they did call for compensation for Sony which -- following threats against movie-goers -- pulled the Christmas Day debut of comedy "The Interview," which lampoons North Korean leader Kim Jong-Un.

"If they want to help here they could admit their culpability and compensate Sony for the damages that they caused," State Department deputy spokeswoman Marie Harf told reporters.

Dyn Research said earlier Monday that Internet connectivity between North Korea and the outside world, never good at the best of times, had begun to show signs of instability over the weekend.

"This is different from short duration outages we have seen in the past," said Zmijewski in an email to AFP.

But he stressed it was impossible to say what had caused the outage. "They could have elected to simply pull the plug or they could have suffered from some sort of failure or attack," he said.

"I wouldn't be surprised if they are absorbing some sort of attack presently," Doug Madory, the director of Internet analysis at Dyn Research, had earlier told the North Korea Tech website.

North Korea's communist authorities have denied being behind the Sony hacking that also led to the release of a slew of embarrassing company emails.

Instead, Pyongyang has called for a joint investigation, and vowed reprisals if the US brings in new sanctions, including putting the country back on the list of state sponsors of terrorism.

The diplomatic row comes as China failed on Monday to block the first-ever UN Security Council meeting on North Korea's dismal rights record after a strong majority of members voted in favor of it.

US envoy Samantha Power said the offer of joint investigation into the hack attack was "absurd" calling on the council to take action against North Korean leaders who have turned the country into a "living nightmare."

- China's hand? -

North Korea, one of the most repressive nations on the planet, has limited access to the worldwide web, with just four networks on the global Internet, compared to 150,000 in the United States.

All of North Korea's routing is done through China Netcom, which is now part of China Unicom, Zmijewski said.

Washington has urged Beijing -- Pyongyang's closest ally -- to help rein in the North's cyber hacking activities, with US Secretary of State John Kerry speaking with Chinese Foreign Minister Wang Yi over the weekend to discuss the problem.

"As North Korea's sole Internet provider, it would be easy for China Unicom to disable North Korea's access," Zmijewski added.

"We have checked from hosts inside of China Unicom's network and can confirm that North Korea is not visible from within China either."

Pyongyang's main Internet presence is through its Uriminzokkiri website, which has Twitter and Flickr feeds and is best known for posting propaganda videos excoriating South Korea and the United States.

Obama had vowed a "proportional" response, saying he saw the Sony hack as an act of "cyber vandalism" not a declaration of war.

But US officials have been tight-lipped about exactly what action they will take, and Harf refused to confirm reports that North Korea's cyberspace was under attack.

The US administration is "discussing a range of options" in response to the Sony hacking, she said.

"As we implement our responses, some will be seen, some may not be seen," she said.

In Beijing, a foreign ministry statement on Monday said the country "opposes cyber attacks and cyber terrorism in all of its forms," without referring directly to North Korea.

Earlier this year the United States charged five Chinese military officials with allegedly hacking US companies for trade secrets.