* Alleged killings occurred between 2013 and 2015
* Mass grave uncovered in February
* Congo rejects accusations that it has ignored allegations
By Joe Bavier
ABIDJAN, June 7 (Reuters) - Human Rights Watch on Tuesday 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.
A Congolese defence ministry official, contacted in Brazzaville, said an investigation was underway and rejected claims it had ignored the allegations.
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, publically executed another two suspected anti-balaka in February 2014, and beat two civilians to death in June 2015.
HRW also said a mass grave found near a base once occupied by Congolese troops in the town of Boali was found to contain the remains of 12 people identified as having been detained by the peacekeepers on March 24, 2014.
"The discovery of 12 bodies is damning evidence of an appalling crime by Congolese peacekeepers, who had been sent to protect people, not prey on them," said HRW Africa researcher Lewis Mudge.
The United Nations took over peacekeeping responsibilities from the A.U. in Central African Republic in September 2014 and has since come under fire for rights violations alleged to have been committed by its soldiers. The HRW said the U.N. force had insisted the Congolese troops implicated in the alleged killings in Boali be sent home and replaced by new units.
"Simply rotating troops out of the Central African Republic with no further consequences sends the message that peacekeepers can get away with murder," Mudge said.
U.N. spokeman Stephane Dujarric said U.N. 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 said it had over the past two years repeatedly contacted Congolese authorities, including President Denis Sassou Nguesso, calling upon them to launch credible investigations and punish those responsible for the abuse.
However, it said no action had been taken.
"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."
Dujarric said the United Nations would "continue to follow up ... with the African Union and Republic of Congo authorities, as it has been doing over the course of the last two years."
He added that it was the duty of local authorities to secure the mass grave site. African Union and Central African officials were not immediately available for comment.
Neither the U.N. nor countries hosting U.N. 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.
(Additional reporting by Louis Charbonneau at the United Nations and Christian Elion in Brazzaville; Editing by Tim Cocks and Richard Balmforth)
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