When Dung Nguyen left Vietnam to work abroad, the 37-year-old said he'd been assured he would be employed by a German company in Serbia, only to have his passport taken away upon arrival at a Chinese-run factory where conditions were dire.

The situation at the factory and the alleged deception used to lure employees has made headlines in Serbia after Nguyen and hundreds of other Vietnamese went on strike this week.

The strike that started on Wednesday was a rare show of defiance by labourers at a Chinese-backed enterprise in the country.

Beijing has invested billions in Serbia and neighbouring Balkan countries in recent years, hoping to expand its economic footprint in central Europe.

Serbia has been quick to cash in on China's interest, as it seeks to court a range of investors amid the ongoing tug of war between the East and West over influence in the Balkans.  

But Belgrade has repeatedly been accused of giving Chinese-owned companies a free hand in how they run their operations.

Critics from civil society, human rights groups and in the media say the government has turned a blind eye to environmental concerns and potential human rights violations.

The Vietnamese workers were employed to build a factory for the Chinese tyre company Linglong in the small northern city of Zrenjanin, considered a centrepiece of Beijing-backed investment in Serbia.

But according to Nguyen, the living and work conditions were untenable and not what he had been promised.

"We are living as if we were in jail... all our passports were kept by the Chinese when we arrived at the airport," Nguyen told AFP in a video message sent from inside the living quarters.

"I cannot talk more as I am afraid my saying would impact others," he added.

Even before the strike, private security guards were posted near the workers' dormitories next to the factory site and journalists including from AFP were prevented from entering the premises.

Human rights organisations A11 and ASTRA published a joint report earlier this week demanding "urgent action" from Serbian authorities.

"A large number of established facts indicate the possibility that workers are victims of human trafficking for the purpose of labour exploitation," it said.

According to the report, the Vietnamese workers had not been provided with heating, electricity or hot water and the facilities lacked adequate infrastructure and sewerage. 

"The conditions were nowhere near suitable for housing human beings," Danilo Curcic, a human rights lawyer from A11 said during an interview with local broadcaster N1 TV.

"I don't think it's an overstatement to say that some people do not keep animals in those conditions." 

Construction workers at the Zrenjanin factory had already staged two strikes within the last six months, according to A11, sparked by unpaid salaries and a lack of food. 

A short documentary aired by N1 this month also showed workers living in cramped conditions inside a makeshift dormitory at the site.

"It is unacceptable that an aspiring EU member state seems to tolerate this on its territory and remains silent on cases of potential forced labour in Europe," Viola von Cramon, a member of the European Parliament for Germany, told AFP.

Linglong said that the Vietnamese workers were not officially employed by the company and had been hired by a Chinese subcontractor.

"Linglong's only obligation to its contractors is to pay them compensation for the work performed under the contract," the company said in a statement.

It added it was planning meetings with subcontractors to "inform them about the values the company upholds" and demanded the workers were transferred to "better accommodation".

It did not respond to an AFP request for further clarification. 

On Friday night, the Vietnamese workers were asked to sign a statement declaring they had "voluntarily and consciously" agreed to work at the site, AFP confirmed from various sources.

"Everybody became anxious and scared, without knowing what the Chinese would do to us," Nguyen said. 

"We dont want to work in the Chinese company. We want to leave. Please help us."

Vietnam's foreign ministry said officials had received no reports of "violence and harassment" at the factory but said it was monitoring the situation. 

In a 2019 case that cast a disturbing light on unscrupulous trafficking networks, 39 Vietnamese migrants were found dead in a refrigerated truck in Britain after it had crossed the Channel from Europe.

Serbian leaders have batted away accusations of malfeasance at Linglong.

Prime Minister Ana Brnabic suggested the incident might have been a conspiracy targeting Chinese investment in the country, after confirming that the Vietnamese workers were being moved to more appropriate accommodation. 

President Aleksandar Vucic has vastly expanded ties with Beijing since coming to power and says the two countries enjoy a "steel friendship".

Serbia was one of the first countries in Europe to receive coronavirus vaccines from China, while Vucic kissed the Chinese flag last year after receiving medical supplies sent by Beijing early on in the pandemic.

Following this week's headlines, the Serbian leader doubled down, saying Chinese investments would continue to be a top priority.

"What do you want, to destroy an investment worth $900 million?" Vucic said Friday.  

"If the Vietnamese need to be helped, we will help. But we will not chase the investors away."