North Korea has intensified its border control and imposed stronger punishment on those who were caught fleeing the repressive country, a report by a South Korean think tank showed Tuesday.

Since 2014, North Korea has raised the level of punishment against people who have sought to escape the nation, according to the white paper on the North's human rights, released by the state-run Korea Institute for National Unification.

The report was written based on interviews with 186 North Korean defectors who arrived in South Korea between late 2014 and last year, it said.

Since 2009, the North's regime has raised surveillance on North Koreans whose family members defected to the South, it said.

"North Korea has strengthened its punishment over those who were caught using mobile phones at areas bordering China and sharply beefed up surveillance near coastal areas," it said.

Since North Korean leader Kim Jong-un took power in late 2011, he has extended the so-called reign of terror in a bid to take a firmer grip on power.

The report said that regardless of the number of attempts made to flee the country, North Korea has imposed forced labor on those caught since 2014.

Besides cracking down on people trying to contact the outside world using mobile phones, the North has made a determined effort to keep out DVDs of South Korean movies and dramas as they could motivate North Koreans to defect, it said.

Meanwhile, the report said that North Korean workers forced to toil overseas are required to send an excessive amount of their wages to the North's regime after working under poor working conditions.

Many North Koreans are working in the logging and construction sectors, mainly in China, Russia and the Middle East, as means of providing dollars to the North's regime, it added.

It did not reveal the number of such workers, but a U.N. report said that North Korea has sent more than 50,000 workers abroad, mainly to China and Russia, as it seeks to obtain dollars to avert economic hardship under heavy United Nations sanctions.

A South Korean civic group estimated that $200 to $300 million is presumed to be sent to North Korea's regime annually.

The white paper said that North Korean overseas workers have to give a "considerable" amount of their wages to the North's Workers' Party of Korea to prove their loyalty.

North Korea has long been regarded as one of the worst human rights violators. Pyongyang has bristled at such criticism, calling it a U.S.-led attempt to topple its regime.

The North does not tolerate dissent, holds hundreds of thousands of people in political prison camps and keeps tight control over outside information.

In December 2015, the U.N. General Assembly adopted a resolution for the second consecutive year that calls for referring the North to the International Criminal Court for human rights violations. (Yonhap)