The growing availability of news media and mobile phones in North Korea probably forced it to admit within hours that its long-range rocket launch last month was a failure, says the US human rights envoy to the country.

Robert King was speaking at the launch of a US government-funded study that says North Koreans now have unprecedented access to foreign media, giving them a more positive impression of the outside world.

Leader Kim Jong-un and his government are losing control of information. Credit:AFP

''The media environment in North Korea has changed and is changing, and with the availability of mobile phones for internal communication, and greater availability of information internally, you can't just say, 'Let's play patriotic songs' so all can tune in,'' Mr King said.

The study, commissioned by the State Department and conducted by consulting group InterMedia says North Korea still has the world's most closed media environment - there is still no public access to the internet - but the government's control of information is receding.

Restrictions that threaten years in prison and hard labour for activities such as watching a South Korean soap opera or listening to foreign news broadcasts have been tightened since the mid-2000s, but are enforced less than in the past, the study says. People remain wary of government inspection teams, but fewer citizens appear to be reporting on each other.

''The state can't count on their citizenry to turn each other in,'' the main author, Nathaniel Kretchun, said.

The study, A Quiet Opening: North Koreans in a Changing Media Environment, is based on research involving several hundred North Korean defectors and refugees during 2010-11. It found nearly half had watched a foreign DVD, the most commonly used type of outside media.