The statement was issued by Human Rights Watch, Amnesty International, and the Memorial Human Rights Center.
According to news reports, assailants shot and killed 27-year-old Umar Israilov on January 13 in Vienna, where he had lived in exile.
Israilov had stated publicly that he had been tortured by Kadyrov and had filed a complaint with the European Court of Human Rights in 2006.
Several days prior to the killing, Israilov complained to Austrian police authorities that he was being followed by unknown individuals.
“We are deeply alarmed about what appears to be another politically motivated killing of a critic of high-level Russian government officials,” said Oleg Orlov, director of Memorial.
“In light of the brutal retaliation inflicted on those who speak out on abuses in Chechnya, Israilov’s actions were particularly courageous, and his killers and those behind them need to be promptly held to account," Orlov said.