POLICE, Poland (NYTIMES)- At an isolated shipyard on Poland's Baltic coast, men in coveralls used welding torches under a cold drizzle, forging an oil tanker for a customer in the Netherlands. The scene was unremarkable, save for the provenance of a dozen of the workers.

"Yes, we are from the Democratic People's Republic of Korea," one of them said. "We have been here quite a while." Then he hurried away, alarm seizing his face.

Four other welders confirmed that they were also from North Korea, the pariah state threatening the United States and much of East Asia with nuclear weapons. They, too, then scampered off.

For decades, North Korea has dispatched labourers to points around the globe, engaging tens of thousands in logging, mining and construction ventures while taking a hefty slice of their earnings. The United States has sought to shut down this enterprise, lobbying other countries to eject the workers and eliminate a source of hard currency for the North Korean economy.

But the continued presence of these workers in Poland - a Nato ally at the heart of the European Union - underscores how difficult it is to fully sever North Korea from the global economy, even as the nation accelerates efforts to build a nuclear missile capable of striking the United States.

In December, the United Nations Security Council adopted a resolution requiring all countries to expel North Korean workers within two years. The resolution, which followed the North's launch of a new intercontinental ballistic missile in November, also imposed a sharp cut in oil shipments to the nation.

Last Thursday, President Donald Trump accused China of allowing fuel to be smuggled into North Korea, saying Beijing had been caught in the act. The assertion came amid reports of secret ship-to-ship transfers in international waters by Chinese and Russian vessels.

China and Russia, which host the majority of North Korea's overseas workers, have long resisted US efforts to impose a global embargo on the nation. Even the European Union agreed only in October to stop renewing work permits for North Koreans.

Poland sent soldiers to fight alongside Americans in Iraq, but is nonetheless one of the few countries still hosting North Korean workers over Washington's objections.

The State Labour Inspectorate, which regulates working conditions at Polish companies, said that perhaps 450 North Koreans remained in the country as of mid-2017, employed by at least 19 companies, including a complex of greenhouses growing tomatoes south of Warsaw.

But The New York Times found North Korean workers at two other businesses - the shipyard in Police, near the German border, and a factory that makes shipping containers in the town of Czluchow, 160km south-west of Gdansk.

TRICKY ISSUE

In Poland, provincial governments issue work permits to foreign labourers, and there is little coordination with national agencies. As a result, no one appears to know precisely how many North Koreans are in Poland or what they are doing.

The Foreign Ministry has urged local governments to stop approving work permits for North Koreans, and new legislation taking effect in January will require them to do so. But until now, the provinces have persisted, illustrating the durability of commercial relationships forged during the Cold War, when Poland was a fellow member of the Communist bloc.

Relations between Poland and North Korea cooled after the fall of the Soviet Union, but Poland remains one of seven European nations to maintain embassies in Pyongyang.

The Times requested information on work permits issued to North Koreans from Poland's 16 provincial governments. Nine responded, reporting that they had given 124 new permits to North Koreans in 2017, and 253 the previous year.

Washington has intensified pressure on countries to stop hosting North Korean labourers, and the list of countries doing so has dropped to perhaps 16, including Austria and several Persian Gulf states, from about 40, according to human rights groups and UN reports.

The Polish government has repeatedly pledged to phase out work permits for North Koreans after negative attention in academic papers and news reports.

But the European Union has not pressed the issue, fearful of ratcheting up tensions over sovereignty issues after Britain's vote to exit the bloc. Poland's right-wing government has bristled at European criticism of its moves to exercise greater control of the courts.

"The EU has been afraid of driving Poland further away," said Remco Breuker, a historian and Korea expert at Leiden University in the Netherlands.

In a warren of streets near the Oder River in Police, a dreary town of 40,000, North Korean workers are often seen walking near train tracks from the Partner shipyard to a grocery store.

Sometimes they squat on the sidewalks in front of their dormitory, smoking cigarettes while braced against a biting wind. In the evenings, they trudge to buy pastries or vegetables. On Sundays, they congregate at a local elementary school for soccer games.

"It's something very unusual," said Pawel Wieczorkowski, deputy director of the local unemployment office. "It's exotic." The workers appear intent on maintaining a low profile. With few exceptions, North Korea's totalitarian government forbids citizens from mixing with outsiders. Those who fall under suspicion can face arrest.

"We are here legally. We pay taxes to the local government," a North Korean worker said after being approached outside the dormitory. Asked about reports that workers have been mistreated, he snapped, "They are all lies!" Then he got in a van and drove away, down a muddy alley.

'FORCED LABOUR'

North Koreans working elsewhere in Poland also keep to themselves.

At the greenhouse complex that employs North Koreans, the workers' dormitory was surrounded by a 7-foot-high concrete wall. But through a crack, a heavyset man in a thick parka could be seen directing six women to wash a blue Ford van in the winter chill.

In Koldowo, a speck of a village some 320km north-west of Warsaw, residents said a group of North Koreans arrived in early 2017 for jobs at Remprodex, a manufacturer of shipping containers in the nearby town of Czluchow.

During their first months, they slept inside empty containers despite the cold, residents said. Later, the workers rented half of a house tucked inside a walled compound.

Remprodex did not respond to questions, and Times reporters were turned away at the estate of the Kociszewscy family, which owns the greenhouses.

The countries hosting North Korean workers have defended the arrangements, arguing that they expose the labourers to the outside world and help them support their families. Conditions back home can be so desperate that some North Koreans pay bribes to get these jobs.

But human rights organisations, North Korean defectors and UN monitors have described the assignments as forced labour because the workers are physically confined, under constant surveillance and deprived of most of their wages.

As many as 147,000 North Koreans now work abroad, according to a recent estimate by the Korea Institute for National Unification in Seoul, and the ruling Workers' Party in Pyongyang is said to seize anywhere from 30 to 80 per cent of each labourer's earnings.

That amounts to a significant revenue source for a regime increasingly pinched by international sanctions - between US$200 million and US$500 million (S$266 million to S$665 million) annually, according to most experts.

The call that would put her in business with North Korea came around 2007, Cecylia Kowalska recalled.

At the time, she ran a company in the port city of Gdansk that supplied electrical and welding services to the shipping and construction industries.

A shipyard in Gdansk needed someone to manage 10 North Korean welders who had worked there in the past, she said. They had been employed through another firm that had struggled to pay them on time.

'TRADE SECRETS'

Kowalska, now 67, said her company, Armex, assumed responsibility for the workers, and then established a relationship with the North Korean partners who had brought them to Poland.

She later began supplying North Korean welders to two other shipyards, run by Crist SA and Nauta SA, both companies that make war vessels for NATO members.

"They were skilled and hardworking," she said of the North Koreans.

Kowalska also served as the legal representative of a company called Wonye that was established to supply workers to factories, shipyards, and fruit and vegetable wholesalers, according to Polish corporate records.

The records identify Wonye's president as a North Korean named Jo Chol-yong.

In the mid-1990s, a man of the same name and birth date worked for a North Korean company controlled by the ruling party department that oversees the nation's nuclear and missile programs, according to a registry of Pyongyang residents smuggled out of North Korea.

Wonye's vice-president is listed as Kang Hong-gu, who appears to have previously served as commander of a unit involved in construction, the 8th Sokdojon Brigade, according to the Pyongyang registry.

Kowalska said she helped establish Wonye in 2015 as a favour to one of her North Korean partners but never took an active role and sold her shares the next year.

According to research by Breuker and his colleagues, Armex received its workers from the Rungrado General Trading Corp, a North Korean supplier of overseas workers sanctioned by the United States in 2016 and accused of funding the department that oversees the nuclear weapons program.

Asked about her partners, Kowalska said she was uncertain of their names and promised to look them up. But she later declined to identify them, saying that doing so would divulge "trade secrets".

Once, she recalled, one of the North Koreans suggested she buy a gift for officials in Pyongyang - a sword. "A sign of our thanks for this partnership," she said. "I thought it was like buying someone flowers."

The sword was later displayed in a hall for gifts to North Korea's leaders and highlighted in a North Korean propaganda video that identified Armex by name.

Kowalska said the partnership proceeded smoothly until three years ago, when a North Korean welder without adequate safety gear burned to death at the Crist shipyard. The accident alarmed Crist's customers, among them a Danish shipbuilder that had employed the Polish shipyard to handle work on a war vessel for Denmark, another Nato member.

Soon afterward, Kowalska said, she stopped hiring North Korean workers "because it became such a sensitive issue". She added that she was now retired and no longer managed North Korean workers.

But her North Korean partners still appear to be active in Poland.

On a recent afternoon, Times reporters spotted two vehicles with Polish license plates parked outside the workers' dormitory at the tomato greenhouses: the van that was being washed and a dark Mercedes sedan.

According to records seen by The Times, the van is registered to Wonye's vice-president, Kang, and the Mercedes to its president, Jo.

Though the European Union maintains extensive labour protections, North Koreans who have worked there describe a parallel reality.

"Our girls lived as if they were in prison," said Kim Tae-san, a North Korean defector who worked in the Czech Republic from 2000 to 2002 supervising 200 young North Korean women in a shoe factory.

CLOSELY MONITORED

He said the women were forced to remain during off-hours in their dormitory, where they attended ideological study sessions and could watch only movies and propaganda documentaries sent from home.

Once a week, he added, they were allowed out to go to the market - but only in groups.

North Koreans sent to work overseas are vetted for political loyalty, but the government also sends minders to watch them. Kim said the workers also "monitored each other".

The women worked six days a week, earning US$150 a month but keeping only about US$25 for food and savings. Their supervisors took the rest, Kim said, spending some of the money on housing but sending most of it back to the authorities in North Korea.

Poles who have worked with North Koreans describe similar conditions. A shipyard worker at Crist, for example, recalled how a North Korean colleague fell ill on the job and was urged by a paramedic to stop working. Instead, the man became frantic, insisting he had to continue.

"This is slave labour," said Agnes Jongerius, a Dutch member of the European Parliament, who has urged European authorities to force Poland to stop admitting North Korean workers.

Kowalska scoffed at allegations of abuse and said the North Koreans she managed enjoyed "a normal life". "They asked us for advice on what to buy their wives and kids," she said. "They liked to buy lingerie for their wives. It was a popular gift, and they would ask us about inexpensive shops."

She said her company paid the workers about US$780 per month. She acknowledged at first that Armex sent a portion of their wages to a North Korean company, but later said she had misspoken and no one took a cut.

As international scrutiny has intensified, the State Labour Inspectorate has vowed to investigate claims of abuse. So far, the agency has found "no signs of forced labour", said Dorota Gorajska, an official responsible for companies that employ foreign workers.

Officials acknowledged, however, that inspections have generally been confined to paperwork and that when interviews are conducted, investigators typically rely on translators provided by employers.

Given North Korea's reputation, does that not taint their findings? An official at the inspectorate, Michal Tyczynski, took a deep breath.

"It's a tricky question," he said. "There is no good answer to this question."