POLICE, Poland — 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 said. “We have been here quite a while.’’ Then he hurried away, alarm seizing his face.

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

For decades, North Korea has dispatched laborers 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 UN 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.

On Thursday, President Trump accused China of allowing fuel to be smuggled into North Korea.

The assertion came amid reports of secret ship-to-ship transfers in international waters by Chinese and Russian vessels.

On Monday, North Korean leader Kim Jong Un said the United States should be aware that his country’s nuclear forces are now a reality, not a threat. But he also struck a conciliatory tone in his New Year’s address, wishing success for the Winter Olympics set to begin in the South in February and suggesting the North may send a delegation to participate.

Kim, wearing a Western-style gray suit and tie, said his country had achieved the historic feat of ‘‘completing’’ its nuclear forces.

‘‘The US should know that the button for nuclear weapons is on my table,’’ he said during the speech, as provisionally translated by the AP. ‘‘The entire area of the US mainland is within our nuclear strike range. . . . The United States can never start a war against me and our country.’’

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

In Poland, the State Labor Inspectorate, which regulates working conditions, said that perhaps 450 North Koreans remained in the country as of mid-2017, employed by at least 19 companies.

Material from the Associated Press was used in this report.