North Korea experienced one of its worst internet outages for several hours, days after United States President Barack Obama vowed a "proportional" response for the hermit country's alleged cyber attack on a Hollywood studio.
The problems began at the weekend, and by yesterday North Korea was completely cut off from the World Wide Web. One expert described its connectivity as "toast".
US-based internet analysts Dyn Research said Pyongyang's four online networks, all connected through Chinese telecom provider China Unicom, had been offline for nine hours and 31 minutes before services resumed yesterday.
The United States declined to comment amid speculation it was hitting back in a new cyber war to protect itself from future hacking assaults. But US State Department spokeswoman Marie Harf said: "As we implement our responses, some will be seen, some may not be seen."
Other possible explanations for North Korea's internet woes included China, through which many of its connections are routed, wanting to restrain its ally by limiting its access. It could also have been a maintenance issue.
Doug Madory, of Dyn Research, said: "I haven't seen such a steady beat of routing instability and outages before. Usually there are isolated blips, not continuous connectivity problems." He added: "North Korea [was] totally down."
Matthew Prince, of US internet company CloudFlare, told the New York Times North Korea's internet was "toast".
Meanwhile, the US demanded North Korea pay for the cost of the recent attack on Sony Pictures which is estimated to have cost the studio up to US$200 million ($258.3 million).
Harf said: "If they want to help here they could admit their culpability and compensate Sony for the damages that they caused."
North Korea is suspected of putting agents in America in the 1990s with missions to attack nuclear facilities in the event of war between the two countries, according to a declassified report.
It reportedly established five "liaison offices" which were intended to "train and infiltrate operatives into the United States", a report from the US Defence Intelligence Agency said.
In the event of hostilities breaking out, covert units were to target "nuclear power plants and major cities" in the US.
The report, compiled in September 2004 and obtained by the Washington Free Beacon, was heavily redacted and classed as an "information report, not finally evaluated intelligence". The source of the information was redacted and it did not say whether the North Korean units were thought to be still active in 2004. At the time, North Korea was motivated to establish a covert presence because it was struggling to develop a missile that could reach the US.