From Reuters, via the Washington Post:

Many North Korean refugees who flee to China every year end up as sex slaves and China often sends them back for punishment, the U.S. ambassador for fighting international slavery said in Beijing on Friday.

John Miller, the director of the U.S. State Department’s Office to Monitor and Combat Trafficking in Persons, said he discussed North Korea in two days of talks with Chinese officials about the modern-day slave trade — many of its victims women and girls forced into prostitution and marriage.