A group of North Koreans sent to work in a Chinese seafood factory last month rioted and occupied the building to protest unpaid wages, Japanese media revealed Sunday. During the protest, they took a factory official hostage and beat a representative of the owning company to death.

The story was broken by Japan’s Yomiuri Shimbun newspaper, which spoke to a source in North Korea about the January 11-14 incident. The source described it as the first large uprising by dissatisfied North Korean workers sold as virtual slaves to foreign companies, and also a warning sign for the regime in Pyongyang that its younger subjects are growing harder to control.

The Chinese factory was variously described as manufacturing apparel, processing seafood, and even – according to Radio Free Asia (RFA) – producing medicine. The facility is located in the Chinese city of Helong, in a development zone that runs along the border with North Korea.

About two thousand North Korean workers were shipped to the factory under a contract with the North Korean Defense Ministry, which wanted its indentured servants to earn Chinese currency that could be brought back to Pyongyang.

North Korean slave workers typically earn a very meager salary, which they are not paid until they return to North Korea. The money they earn is held by the Communist tyranny, which keeps most of it for “expenses” and “contributions” to the motherland.

The North Korean workers in Helong reportedly heard a rumor that previous work crews were paid nothing when they returned home because the regime decided to keep all of their salaries to finance “war preparations” against South Korea and the United States. Rumors flew around North Korea that most of the money was embezzled by high-ranking officials.

A riot broke out on January 11, during which the North Korean workers, many of them female former soldiers, managed to seize control of the factory and take hostages. Pyongyang dispatched secret police to deal with them and alerted the Chinese consulate, but the workers refused to let anyone enter the factory.

At some point during the worker revolt, the workers physically assaulted one of their hostages, who was described in media accounts as either the manager of the factory or a representative of the company that owns it. The occupation of the factory ended on January 14 after the workers beat this individual to death.

According to Yomiuri Shimbun’s source, the workers were “temporarily placated” when the North Korean regime agreed to pay their wages, but then some 200 of the 2,000 workers were seized by the secret police and dragged back to a North Korean “political prison camp” to receive “severe punishment.”

North Korean dictator Kim Jong-un and his top henchmen are reportedly “in shock” over the incident, which was one of the most forceful acts of defiance against Kim’s rule since he took power in 2011. The regime is said to be worried that other work crews sent abroad will hear of the Helong incident and contemplate their own acts of defiance.

“It was the first large-scale protest by North Korean workers in China, and it brought to the surface the anti-authoritarianism of North Korean youth who refuse to accept slavery,” said Yomiuri Shimbun.

RFA noted that North Korean defector Ko Young-hwan, who now works for the South Korean Unification Ministry, said last month that North Korean workers in China did indeed conduct “strikes and riots” as word of the January 11 uprising spread, but they seem to have subsided a few days later. Ko anticipated the unrest might resume because “funds to pay back wage arrears have dried up.”