North Korea has called US President Barack Obama a "monkey" and blamed Washington for internet outages that it has experienced during a confrontation with the United States over the hacking of the film studio Sony Pictures.

The National Defence Commission, the North's ruling body, chaired by state leader Kim Jong-un, said Obama was responsible for Sony's belated decision to release the action comedy The Interview, which depicts a plot to assassinate Kim.

"Obama always goes reckless in words and deeds like a monkey in a tropical forest," an unnamed spokesman for the commission said in a statement carried by the official KCNA news agency, using a term seemingly designed to cause racial offence.

Sony cancelled the release of the film when large cinema chains refused to screen it following threats of violence from hackers, but then put it out on limited release after Obama said Sony was caving in to North Korean pressure.

Obama promised retaliation against North Korea, but did not specify what form it would take.

North Korea's main internet sites suffered intermittent disruptions last week, including a complete outage of nearly nine hours, before links were largely restored on Tuesday.

But its internet and 3G mobile networks were paralysed again on Friday evening, China's official Xinhua news agency reported, and the North Korean government blamed the United States for systemic instability in the country's networks.

In its statement, the North again rejected an accusation by the US Federal Bureau of Investigation that North Korea was behind the cyber-attack on Sony Pictures, and demanded the United States produce evidence to support its allegation.

The National Defence Commission also dismissed US denials of involvement in North Korea's internet outages.

"The United States, with its large physical size, and oblivious to the shame of playing hide and seek as children with runny noses, has begun disrupting the internet operations of the main media outlets of our republic," it said.

In a separate commentary, the North denied any role in cyber-attacks on South Korea's nuclear power plant operator, describing the suggestion that it had done so as part of a "smear campaign" by unpopular South Korean leaders.