North Korea has demanded former detainee and Christian missionary Kenneth Bae stop "babbling" about his time in prison, warning that if he does not do so, it will not negotiate with the US over two American citizens it is still holding.

"As long as Kenneth Bae continues his babbling, we will not proceed with any compromise or negotiations with the United States on the subject of American criminals, and there will certainly not be any such thing as humanitarian action," the KCNA news agency said.

"If Bae continues, US criminals held in our country will be in the pitiful state of never being able to set foot in their homeland once again".

Bae, a Korean-American, was the longest held US citizen in North Korea since the Korean War. He was running a legal tour company in North Korea when he was sentenced to hard labour for 15 years in April 2013 following accusations that he was committing hostile acts against the state and encouraging citizens to work against the government.

He was sent to a camp for foreign detainees where about 30 guards kept watch over him as their sole prisoner but was released in November 2014.

He has released a book, Not Forgotten, about his experiences, and has said he was accused of trying to overthrow the North Korean government through his Christian worship and by spreading Western ideas.

Criticised over its human rights record for years, North Korea has made use of detained Americans in the past to extract high-profile visits from the United States, with which it has no formal diplomatic relations.

Pyongyang is now holding two US citizens, both of whom it has tried and sentenced to hard labour.

In March, Otto Warmbier, a 21-year-old student of the University of Virginia, was sentenced to 15 years' hard labour for trying to steal a propaganda banner bearing the name of former leader Kim Jong Il.

In April, a North Korean court convicted Korean-American missionary Kim Dong Chul of crimes against the state and sentenced him to 10 years' hard labour.

Last year, Canadian missionary Hyeon Soo Lim was sentenced to hard labour for life for subversion of the state.

North Korea has consistently been named the worst country in the world to be a Christian. Under dictator Kim Jong-un, the government maintains absolute control through the systematic repression of its citizens. According to Aid to the Church in Need, of the 400,000-500,000 Christian population in North Korea, at least 50,000 are thought to be in hard labour camps, while tens of thousands of citizens, including many Christians, have defected to countries such as neighbouring South Korea, China, Mongolia and Russia.

Addtional reporting by Reuters.