Nuclear-armed North Korea has long made a fortune from an army of citizens it sends abroad to work, mostly in neighbouring China and Russia.

A 2017 UN Security Council resolution, supported by China, required countries to deport North Korean workers to prevent them earning foreign currency for Pyongyang's nuclear and ballistic missile programmes -- but analysts have accused Beijing and Moscow of circumventing the measures.

And Monday's report by the London-based Environmental Justice Foundation (EJF) alleges widespread abuses of North Korean workers at sea, in violation of sanctions.

"North Koreans onboard were forced to work for as many as 10 years at sea -- in some instances without ever stepping foot on land," the report said.

"This would constitute forced labour of a magnitude that surpasses much of that witnessed in a global fishing industry already replete with abuse."

The claims were based on interviews with over a dozen Indonesian and Filipino crew members who worked onboard Chinese tuna longliners in the Indian Ocean between 2019 and 2024.

"They never communicated with their wives or others while at sea as they were not allowed to bring a mobile phone," one crew member was quoted as saying.

Another said some North Koreans had worked on the vessel for "seven years, or eight years", adding: "They were not given permission to go home by their government."

- 'Tainted by modern slavery' -

The report also said that vessels carrying the North Koreans were involved in shark finning and capturing large marine animals, such as dolphins, and potentially supplied markets in the European Union, the United Kingdom, Japan, South Korea and Taiwan.

In one image, a dolphin can be seen with its head cut off.

"The impact of this situation is felt around the world: fish caught by this illegal labour force reaches seafood markets around the world," Steve Trent, CEO and founder of the EJF, said in a statement.

"China bears the brunt of the burden, but when products tainted by modern slavery end up on our plates, it is clear that flag states and regulators must also take full responsibility."

The US State Department said last year that an estimated 20,000 to 100,000 North Koreans work in China, primarily in restaurants and factories.

North Korea withholds up to 90 percent of wages from its overseas workers and imposes forced labour conditions on them, according to a State Department report.