Isolated North Korea is developing sophisticated tools to digitally spy on its citizens, who are increasingly using mobile devices to connect to each other, a U.S. government-funded report released on Wednesday says.

The report, "Compromising Connectivity", says North Korea has allowed the growing use of mobile phones and domestic internet access in return for the detailed information the network feeds Pyongyang's surveillance state.

"By giving citizens new networked technologies like mobile phones and tablets, the government is able to automatically censor unsanctioned content and observe everything citizens are doing on their devices remotely," Nat Kretchun, the report's lead author, told Reuters.

"The authorities have found ingenious ways to turn those new technologies against the North Korean people who are adopting them," said Kretchun, a researcher at InterMedia, the Washington-based organisation that produced the report.

North Korea's strategies for keeping tabs on its citizens have been in the limelight since last month's dramatic killing of leader Kim Jong Un's estranged half-brother at an airport in the Malaysian capital.

Access to outside information is tightly controlled in North Korea. South Korean television shows and Hollywood films are often shared in a genuinely social network of people trading or swapping files via Bluetooth, or small and easily-concealed USB sticks.

To fight such use, North Korea has rolled out mandatory software updates to mobile devices on its network that actively seek out and delete illegal foreign media files, the report says.

On North Korea's own "Red Star" computer operating system, software scans text documents for specific words or phrases deemed unfavourable by the regime and deletes them.

TEXTS FROM KIM

Many North Koreans near neighbouring China secretly use cheap mobile phones on Chinese networks to bypass state control and speak to foreign contacts or access outside information.

But North Korea's official mobile phone network, Koryolink, has around 3 million subscribers in a country of 24 million, Reuters reported in 2015. 

No new figures have been released, and Egypt's Orascom Telecom, which owns Koryolink as a joint venture with the North Korean state, said later that year it had effectively lost control of its Pyongyang business.

Network calls and data are "subject to controls and surveillance by at least eight ministries and organisations between the party, state and army," the report says, including the feared State Security Department, or "bowibu", North Korea's secret police.

The report, based largely on interviews with North Korean defectors, says Koryolink subscribers receive propaganda via text messages, such as reports of public appearances by leader Kim Jong Un.

Bowibu officers in provincial jurisdictions use covertly gathered data to decide whether to approve citizens' applications to buy a legal mobile phone, it added.