Chadian soldiers killed 30 civilians and seriously wounded more than 300 in an indiscriminate attack on a market on March 29 in Bangui, capital of the Central African Republic, a spokesman for the UN human rights office said.
Rupert Colville, reporting on a preliminary investigation that had interviewed survivors in hospital and visited the scene, said on Friday a convoy of pick-up trucks from Chad's regular army, not part of an African Union peacekeeping force, entered the market in the capital's PK12 district and started firing.
"As soon as the convoy of the Chadian national army reached the PK12 market area around 3:00 p.m., it allegedly opened fire on the population without any provocation. As people fled in all directions in panic, the soldiers continued to fire indiscriminately," Colville told a news briefing in Geneva, according to the Reuters news agency.
"At the time of the shooting, the market was full of people, including many young women and girls buying and selling produce."
The attack appeared to have stopped when Congolese peacekeepers arrived on the scene, he said.
Previously local officials and aid workers had put the death toll at at least 10, with 30 wounded.
Several sources said that the Chadian forces had entered Bangui to extract remaining Chadians and other Muslim inhabitants in order to save them from attacks by the anti-balaka Christian militia, Colville said.
The shooting was the latest in a string of violent incidents involving Chadian troops.
Chad said on Thursday it would withdraw its troops from the African Union peacekeeping mission and Colville said he hoped their departure would also prevent further incursions by troops travelling directly from Chad.
Chadian soldiers have been at the heart of African efforts to stabilise CAR but its forces have been accused of siding with the mainly Muslim Seleka rebels whose seizure of power last year sparked tit-for-tat violence with Christian militia.