A Christian missionary detained for two years in North Korea urged American citizens still held in the reclusive country to remain hopeful.

Speaking at a briefing in Washington on Wednesday, Kenneth Bae, 47, also said the international community should remember the suffering of North Korea's citizens.

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.

Bae said he had to shovel coal, perform farm chores and dig the earth.

He was released in November 2014 when US Director of National Intelligence James Clapper made a secret visit to North Korea and came back with Bae and fellow American Matthew Miller.

US citizens Kim Dong Chul and Otto Warmbler are both currently detained by North Korea, and have been sentenced to hard labour. Chul, a Christian missionary, has been accused of subversion and Warmbler of trying to steal a propaganda banner.

"Continue to have hope in the US government that they are doing everything they can to secure your release and also just take one day at a time," Bae urged them.

He has recently released the book Not Forgotten about his experience in North Korea, and on Wednesday described what led to his arrest.

Bae said that on his 18th trip leading tours in the country, North Korean authorities discovered his hard-drive had Western media coverage of such topics as the country's 1990s famine, material which he said was loaded by mistake.

Bae said he was accused of trying to overthrow the government through his Christian worship and by spreading Western ideas.

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.

Rev Hyeon Soo Lim, a 61-year-old Christian missionary from Canada, has been held in Pyongyang since February 2015 after disappearing during a humanitarian trip to the country.

He was sentended to life in prison and hard labour in December after being charged with 'crimes against the state' by a North Korean court.

His family said his trips to North Korea – of which he had undertaken more than 100 since 1997 – were never politically motivated, but the government does not tolerate any missionary or religious activity deemed to threaten the state's supremacy.

Additional reporting by Reuters.