A new human rights report warns that North Korea’s decades-old practice of dispatching workers to Russia has survived U.N. sanctions to become even more exploitative, funneling millions of dollars to weapons programs while subjecting workers to slavery-like conditions.

The study, released by the Database Center for North Korean Human Rights (NKDB) on Monday, estimates that roughly 15,000 North Koreans are currently employed in Russia, despite U.N. Security Council resolutions mandating their repatriation by the end of 2019.

Researchers say Pyongyang and Moscow have engineered a state-led “ecosystem of corruption” that relies on fraudulent student visasin place of work permits,