Pyongyang is reportedly detaining in concentration camps North Korean nationals who were repatriated from China following their failed attempt to escape to a third country.

Japan's Tokyo Shimbun newspaper said North Korean authorities are also forcing the would-be escapees to take an "oath of silence" in which they pledge to remain silent about what they have seen or heard during their time in China.

The report, quoting a number of North Korean defectors, said Thursday that after being interrogated by state security authorities, repatriated persons were detained for as long as six months and subjected to forced labor.

The daily also said security along the Sino-North Korean border has been strengthened in a bid to crack down on North Koreans attempting to flee their repressive homeland.