North Korea is clamping down on mobile phones and long distance telephone calls to prevent the spread of news about a worsening food crisis, according to the United Nations investigator on human rights for the isolated communist country.

In a report to the UN General Assembly, Vitit Muntarbhorn, a Thai law professor who has never been allowed to visit North Korea, said that its government is using public executions as a means of intimidating the population, and using spies to infiltrate and expose religious communities.

His report came two days after the World Food Programme said that two thirds of North Koreans do not have enough to eat, in the country’s worst crisis since as many as three million people died of famine a decade ago.

“Sadly, even though the harvest was getting better, we have had devastating floods in 2006 and 2007,” Mr Muntarbhorn said in New York. “Over the past year we have had very worrying information of a very chronic food shortage.”

He acknowledged that the government of Kim Jong Il, North Korea’s supreme leader, has allowed access by international agencies to areas damaged by floods in 2007, but described the overall human rights situation as “grave”.

“Particularly disconcerting is the use of public executions to intimidate the public,” he said. “This is despite various law reforms in 2004 and 2005, which claim to have improved the criminal law framework and related sanctions.”

Available food is disproportionately directed to the political elite, the media is controlled by the state, there is no political participation, and dissidents and those with religious faith are persecuted, as well as those who return to North Korea after illicitly leaving the country across the Chinese border.

“Over the past year, we have had reports of more severe sanctions against returnees,” he said. “It does not take very much for the Democratic People’s Republic of Korea not to punish returnees.”

He added: “It is reported that security personnel are ever-present among religious congregations to report on their activities and that the authorities use various persons disguised as religious personnel to monitor religious practices.”

“Some inhabitants watch clandestinely video and TV programs from the south, but in 2008 there were reports of crackdowns on South Korean videos."

Mr Muntarbhorn decried severe constraints on civil and political rights in North Korea, citing reports of "a clampdown on cellphone and long-distance telephone calls to prevent people from reporting on food shortages".

Recent visitors to the country report that North Koreans are no longer allowed to use mobile phones at all. The few foreign tourists who travel there are made to surrender their mobiles on arrival; these are then sealed and returned only when departing the country.

Government officials blame cases of "spying" for the crackdown. There are however reports that a Korean mobile phone service, which would not be able to communicate with the outside world, is being planned for a possible launch early next year.

On Tuesday, WFP announced that some 2.7 million people on North Korea's west coast will run out of food in October, and that, because of the worsening food situation, it was increasing from 1.9 million to 6.5 million the population which it seeks to help with food aid.

“Some areas of the north-eastern provinces in the country have become extremely vulnerable, facing a situation of a humanitarian emergency,” the organisation’s programme director for North Korea said. “We have reached a very critical level and we shouldn't wait for another starvation before ringing the alarm bells.”

Rumours have been swirling that Kim Jong Il, the supreme leader, has been treated for serious illness, although this has been officially denied and there is no way to independently confirm either claim. Whatever the truth, the Government is still in firm control of the country and its grip on the lives of citizens shows no signs of easing.

Money Central: the 10 most decadent dictators

Times Recommends

• UN may halt 400-year-old 'freedom of the seas'
• Where the Rosetta belongs can’t be set in stone
• Gas deal on hold, but Ukraine may pay in the end