The pastor made the claim in "The Realities of Religious Oppression in North Korea," a 24-page booklet he recently wrote based on North Korean defectors' testimonies and U.N. reports on North Korean human rights.

"Current secret religious activities are limited to groups of two or three people reading the Bible, praying and singing hymns," he said.

A mass gathering or preaching is impossible, and those who are found to be involved in such activities are sure to be sent to political prison camps or executed, he said.

In 2006, Son Jong-nam, a North Korean rocket researcher, was sentenced to death for having contact with Christian groups in China and carrying out proselytizing activities in the North.

Some 40 residents in South Pyongan Province and 43 underground Christians in North Hamkyong and Ryanggang provinces were publicly executed in 2010 and 2011, respectively, he said.

Of late, the North is further stepping up crackdowns on secret religious activities, calling religion "opium that paralyzes revolutionary consciousness," he said.

"North Korea is the tomb of religious persons," he said, adding that he published the booklet to use it as a testimony material to take North Korean leader Kim to international court. (Yonhap)