A group of Indian laborers have been trapped in the Central African country since their employer - an India-based timber company - confiscated their identity documents, according to "credible information" received by U.N. human rights experts.
The workers have allegedly not been provided with written contracts or valid work visas, are required to do excessive overtime with no weekly rest, and are not paid regularly, said the U.N. Working Group on Business and Human Rights.
Indian government sources said authorities were looking into the matter, and had notified its high commission in Gabon.
Gabon's labor ministry and government spokeswoman did not immediately respond to a request for comment.
"These precarious working conditions, restrictions on their freedom of movement and confiscation of their identity documents by the company may amount to contemporary forms of slavery and to human trafficking," the U.N. experts said in a statement.
The United Nations said they had been in contact with India, Gabon, and the company, and urged all to look into the issue.
The company named in the U.N. statement - India-based timber firm Accurate Industries - could not be reached for comment.
The 40 workers - some of whom were interviewed by the U.N. experts by phone following a tipoff about their situation from within Gabon - are likely just the tip of the iceberg, according to Surya Deva, chairperson of the U.N. working group.
"We do not fully know how many other companies and how many other workers are there, but based on the information we have it seems this is a pattern," Deva said. "There could be other companies doing something similar."
The United Nations said the workers were based in the Gabon Special Economic Zone in Nkok, which says on its website it is "the biggest furniture manufacturing cluster in Central Africa."
Special economic zones are manufacturing areas set up to attract foreign investment with tax exemptions and flexible labor laws, yet they risk becoming "black holes for human rights abuses," said Deva.
"This case we felt is not only serious but also reflective of the wider issues in special economic zones all over the world including in Africa and Asia," he told the Thomson Reuters Foundation. "That's why we thought we should highlight it."About 23 Indian firms - mainly in the timber sector - were granted licenses to set up units in Gabon's special economic zone in 2016, according to the Indian foreign ministry website.