North Korean teenagers and twenty-somethings are following the government’s call to leave home for coal mines and construction sites, and the country’s leader Kim Jong Un reportedly thanked them for their sacrifices in a new letter released in state media on Sunday.

Kim also used the opportunity to hit out at the U.S., saying “the imperialists' anti-DPRK attempt to degenerate and undermine the ranks of our young people” would be in vain.