International sanctions and a Chinese ban on imports of North Korean coal in February after a series of missile tests have steadily squeezed Pyongyang’s other sources of foreign revenue. That has left the export of labor, along with a string of state-run restaurants and other small businesses in Vladivostok and elsewhere, as one of the regime’s shrinking list of ways to generate hard currency.

To prevent them from seeking refuge in South Korea, North Korean laborers are forced to live together in cramped dormitories scattered around the outskirts of Vladivostok and prohibited from contacting Russians and other foreigners outside work.

The boom in North Korean labor exports to Russia coincides with an expansion of other links between the two countries, including a recent surge in Russian coal exports and the start in May of a new ferry service twice a week between Vladivostok and Rason, a special economic zone on the east coast of North Korea.

In April last year, just months after North Korea announced that it had tested a “miniaturized hydrogen bomb,” Russian and North Korean officials gathered south of Vladivostok to celebrate the reopening of Kim Il-sung House, a wooden building dedicated to the memory of the dictator. It had been rebuilt, at Russia’s expense, after a fire.

The links with Russia are still far less extensive than those North Korea has with China, its principal foreign backer, and do not appear to violate sanctions imposed — with the Russian government’s support — by the United Nations. But they have nonetheless raised eyebrows in the United States and Japan, which want to tighten the economic and diplomatic vise on Pyongyang.

Russian coal exports to North Korea more than tripled to $28.4 million in the first quarter of this year from $7.5 million in the same period in 2014, indicating that Moscow would most likely object to any efforts by Washington to widen United Nations economic sanctions.

Why North Korea would sharply increase coal imports is a mystery, as it has plenty of coal. A bigger mystery is the business rationale behind the new ferry service to North Korea, started last month by a private Russian company, InvestStroyTrest, at a time when few Russians want to travel to North Korea and even fewer North Koreans, aside from laborers, visit Russia.