State media have said that North Korea would "crumble" if such foreign influence is allowed to proliferate.
"There are real fears that these strict measures will far outlast the pandemic," said Sokeel Park, of Liberty in North Korea, which supports defectors.
U.S.-based Human Rights Watch (HRW) said its interviews with North Koreans who left after 2014, or still have contacts there, suggest that while Kim opened up the economy, illegal border crossings became almost impossible, corrupt practices were normalized, and government demands for unpaid labour rose.
"Just like those of his father and grandfather, Kim Jong Un's rule is based on brutality, fear, and repression, instigating systematic rights violations, economic hardship, and possible famine," HRW senior Korea researcher Lina Yoon said in a statement.