Human Rights Watch has accused soldiers from the Republic of Congo of killing 18 people, including women and children, while serving as United Nations and African Union peacekeepers in Central African Republic.
Central African Republic descended into chaos in March 2013 when predominantly Muslim Seleka rebels seized power, triggering reprisals by "anti-balaka" Christian militias.
The New York-based rights group said Congolese soldiers tortured two anti-balaka leaders to death in December 2013, publicly executed another two suspected anti-balaka in February 2014, and beat two civilians to death in June 2015.
HRW said a mass grave found near a base once occupied by Congolese troops in the town of Boali contained the remains of 12 people who had been detained by the peacekeepers in March 2014.
The UN took over peacekeeping responsibilities from the AU in Central African Republic in September 2014 and has come under fire for rights violations alleged to have been committed by its soldiers.
UN spokesman Stephane Dujarric said UN human rights officials investigated the allegations and handed their findings over to local authorities.
At least 13 people, including five women, one of whom was six months pregnant, and two children, were arrested at the home of a local anti-balaka leader after a clash that resulted in the death of a Congolese soldier, HRW said.
That night witnesses interviewed by HRW heard screams and two rounds of gunfire from the Congolese base.
The peacekeepers later warned local residents to avoid a nearby area, claiming it had been mined, HRW said.
A local charity excavated the site in February and the victims were identified by their clothing.
Human Rights Watch over the past two years has repeatedly contacted Congolese authorities, including President Denis Sassou Nguesso, asking for a credible investigation and to punish those responsible.
"Congo is cooperating with the United Nations to verify the allegations against its troops," Congo Defence Ministry spokesman Romain Oba said, rejecting the accusation it had failed to act. "We are waiting for the results."
Neither the UN nor countries hosting UN missions have the authority to prosecute foreign peacekeepers.
Punishment is the responsibility of troop contributing countries, but critics claim they often fail to pursue allegations.