In a statement, Human Rights Watch (HRW) alleged that Shukhoranjan Bali, a prime witness in the Bangladesh War Crime trials was abducted and forced to cross the Indian border.
He was later arrested in India for illegally entering the country and has been languishing in a Kolkata jail since, the HRW said.
"The apparent abduction of a witness in a trial at the ICT is a cause for serious concern about the conduct of the prosecution, judges and government.
Bali was due to appear to give evidence as a defense witness before the ICT, a court expressly set up to try people suspected of war crimes during Bangladesh's 1971 war of Independence. He had previously been listed as a prosecution witness.
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Bali, he said, was expected to counter prosecution allegations about the involvement of Delwar Hossain Sayedee in the 1971 murder of his brother.
Saydeee has since been sentenced to be hanged, in part for the murder of Bali's brother.
In April, Bali was sentenced by an Indian court to 110 days in jail for entering the country illegally.
He has already completed his term but is still in jail.
In its statement, Human Rights Watch said that India should not return Bali to Bangladesh until he is interviewed by the Indian office of the United Nations High Commissioner for Refugees (UNHCR), which can determine if he wishes to claim asylum and whether he is a refugee.
"There is a real risk to Bali if he is returned to Bangladesh, as he could expose those involved in his abduction.
Bali needs access to an independent lawyer and UNHCR so that he can make an informed decision about whether it is safe for him to return to Bangladesh," Adams said.