North Korea, Eritrea and Mauritania have the highest prevalence of modern slavery in the world, according to the 2023 Global Slavery Index published Wednesday, which noted a "worsening" situation globally since its last survey five years earlier.

The report said an estimated 50 million people were "living in situations of modern slavery" in 2021, an increase of 10 million over 2016, when the problem was last measured.

The figure includes some 28 million people in forced labor and 22 million living in forced marriage.

The situation is worsening "against a backdrop of increasing and more complex armed conflicts, widespread environmental degradation" and impacts from the coronavirus pandemic, among other factors, the investigation said.

Compiled by the human rights charity Walk Free, the report defines modern slavery as encompassing "forced labor, forced or servile marriage, debt bondage, forced commercial sexual exploitation, human trafficking, slavery-like practices, and the sale and exploitation of children."

Slavery's core principle entails "the systematic removal of a person’s freedom" -- from the right to accept or refuse labor to the liberty to determine if, when and whom to marry.

By this benchmark, reclusive and authoritarian North Korea has the highest prevalence of modern slavery (104.6 per 1,000 population), according to the report.

It is followed by Eritrea (90.3) and Mauritania (32), which in 1981 became the last country in the world to make hereditary slavery illegal.

The 10 countries with the highest prevalence of modern slavery have some common characteristics, including "limited protections for civil liberties and human rights".

Many of the countries are in "volatile" regions experiencing conflict or political instability, or home to a large population of "vulnerable people" such as refugees or migrant workers.