A global letter-writing campaign has been launched to call for the release of a Christian man imprisoned in North Korea for the last six years. 

Voice of the Martyrs (VOM) is asking people to write a letter on behalf of Jang Moon Seok, an ethnic Korean Chinese deacon from Changbai, China, who is also known by his Chinese name of Zhang Wen Shi.

He ministered to North Koreans along the Chinese-North Korean border by providing food and medicine, and also sharing his faith, before being abducted from China in 2014 and sentenced to 15 years in prison, VOM reports.

VOM's Korea representative, Dr Hyun Sook Foley, believes Jang was imprisoned by the North Korean authorities in a bid to find out more about VOM's ministry there and its work with Korean-Chinese pastor Pastor Han Chung-Ryeol, who was killed in Changbai around a year after Jang's arrest.

Dr Foley told International Christian Concern that Jang "is a simple man who never did anything political".

"He just helped North Korean people for many years. That should never be a crime, and Christians should join together to help Deacon Jang and his family," she said.

VOM said that while Jang and Han engaged in ministry at the border, their "goal was always to see North Koreans return home".

"Near Changbai, North Koreans gather herbs on the North Korean side of the mountain and then take them into Changbai to sell at the market and bring the money back with them to North Korea," the organisation said.

"Deacon Jang regularly hosted these visiting North Koreans for days and weeks at a time before they returned to North Korea, giving them warm clothing, feeding them and providing supplies they might need for their return to North Korea.

"He saw this as his Christian duty to welcome the stranger, clothe the naked and care for the sick. As a believer, he also shared about his faith to those who were willing. A number of these North Koreans accepted the message and became Christians." 

North Korea is one of the most hostile places to be a Christian on earth.  Open Doors ranks it number one on its list of the top 50 countries where Christians suffer the worst persecution.