Human Rights Watch urged Niger to explain its decision to extradite a son of late Libyan dictator Moamer Kadhafi on Thursday.
"The authorities in Niger should explain why they were convinced that Saadi Kadhafi would not be mistreated and would get a fair trial," said Sarah Leah Whitson, HRW's Middle East and North Africa director.
The New York-based group also pointed to the responsibilities of the Libyan authorities.
"Libya has the responsibility to see that Saadi Khadafi gets his full due process rights. They should protect him from torture and cruel, inhuman or degrading treatment; allow him visits from legal counsel, his family and medical personnel; and bring him promptly before a judge," it said.
Kadhafi, who is accused of murder and repression during the 2011 uprising that ended his father's rule, is in the custody of the judiciary's police, the Libyan government said.
Tripoli had repeatedly called for Saadi's extradition from Niger, which granted him asylum in 2011 on "humanitarian" grounds saying it had insufficient guarantees he would have a fair trial.
Tripoli charged he was sowing sedition from exile.
Niger said Thursday it handed over Kadhafi because it no longer felt he would face the risk of extrajudicial killing, and because it wanted to improve ties with Tripoli.
Marou Amadou, the justice minister and government spokesman, said the political situation had "changed" in Libya.
He added that Niger had tried to find another country to take Kadhafi, but "we didn't find any candidate."
But a group of NGOs in Niger condemned the extradition, warning that "the life of Saadi Kadhafi is under threat in Libya, which is a non-state with no security."