North Korea appears to have suffered a second internet blackout, following an outage that saw large parts of the internet unavailable for hours.
Internet performance measuring firm Dyn Research said on its Twitter feed the second internet cut was restored less than an hour later.
"Internet of North Korea down again at 15.41 UTC. Second blackout since last night's restoration of service," the US company said.
Speculation is rife about the source of the blackouts, following a cyber security row with the US.
North Korea has denied it was behind a cyber attack on Sony Pictures and has vowed to hit back against any US 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. The movie studio initially withdrew the film, to criticism from Hollywood and Washington, but today said it would have a limited release in US theatres.
The US requested China's help last Thursday, asking it to shut down servers and routers used by North Korea that run through Chinese networks. 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.
However China has denounced reports suggesting it was responsible for the North Korea outage. US officials have not commented on any possible American involvement.
North Korea is one of the most isolated nations in the world, and the effects of the Internet cut 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.
Doug Madory, director of Internet analysis at Dyn Research, said the North Korea outage may be the result of a software glitch, but a cyber attack could also be the cause.
Other experts said it was possible North Korea was attacked by hackers using a botnet, a cluster of infected computers controlled remotely.
South Korea nuclear data 'stolen'
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.
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.