For years North Korea generated hard currency by illicit means, such as arms sales, drug smuggling and counterfeiting US dollars.

But with United Nations sanctions biting, the cash-strapped regime began tapping a new source of foreign cash — slave labour.

North Korea began sending thousands of its own citizens abroad to prop up the regime.

For Rim Il it was a dream job, a chance to earn $120 a month and eat three meals a day.

At the height of the North Korean famine in the late 1990s he leapt at the chance. Now safe in South Korea Mr Il can tell his story.

"There was plenty of rice and even soup with meat. In North Korea this was unimaginable," he said.

But his job as a carpenter in Kuwait soon turned into a nightmare when he began to work 16 hours a day and was imprisoned in a compound.

He never saw any of his wage, because it went straight back to the regime.

"Looking back I can [see] we were treated like beasts not human beings, we basically weren't human," he said.

'90,000 North Koreans performing slave labour abroad'

North Korean leader Kim Jung-un has doubled the size of the foreign labour program to fund his pet building projects and the pariah state's nuclear program.

North Koreans now toil in 40 countries. Some work in mines in Mongolia, others in Chinese textile factories, many more on construction projects in the Middle East.

Russia takes the most — 25,000 workers.

North Korean Watch in Seoul has been investigating and wants the United Nations to take action.

"Since Kim Jong-un came to power, slave labour has exploded," the organisation's executive director, Myeong Chul Ahn, said.

"We estimate there are about 90,000 and that brings roughly $US2 billion a year to the regime."

North Korean leader Kim Jung-un has doubled the size of the foreign labour program. ( AFP/KCNA )

The ABC spoke with three North Korean men who recently defected to South Korea.

They worked in Siberian logging camps, working long hours in freezing conditions, and for that they were lucky to get 10 per cent of their wage.

The men, who wanted to remain unidentified, told the ABC they only had basic tools and no safety equipment.

They said many of their co-workers died but they could not escape because the regime held family members back in North Korea for ransom.

Researcher Seung ju Lee wrote a book about the men's experiences and said the international community could help.

"While the international community can't do anything about human rights violations inside North Korea, these foreign workers can and should be protected by the host country's labour laws," he said.

Under international pressure, Qatar has sent home some of the North Korean workers who were building the infrastructure for the football World Cup in 2022.