At least 140 of the November deaths are attributable to Uganda's security forces while 15 of the victims were security personnel, Maria Burnett, an associate director in charge of Africa at Human Rights Watch, told The Associated Press. Children as young as 4 or 5 were among those killed, with at least 15 still missing.
Kasese is about 340 kilometers (211 miles) west of the Ugandan capital, Kampala. The area is a hotbed of opposition to President Yoweri Museveni. Museveni has ruled Uganda since 1986, and people in Kasese often complain of marginalisation by the central government.
The clashes followed tensions between Museveni's government and Charles Wesley Mumbere, the leader of a tiny mountain kingdom known as the Rwenzururu. Tribal monarchs in Uganda have only ceremonial powers but are influential among their subjects.
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That account differs from the government's version of events, that fewer than 100 were killed, and will put pressure on the military and the police to investigate.
The government says Mumbere led armed tribesmen who threatened state security in a series of violent encounters near Uganda's border with Congo. The government has arrested and charged scores with murder, treason and terrorism. None of the security personnel involved in the clashes has been charged.
"Let them present the evidence to us, not just writing a report and you think it is fact," he said.
Human Rights Watch said it interviewed more than 95 people in Kasese, including many families of those killed, and reviewed video and photographs of the events.
The rights group said the king's men, armed with tools like machetes, would not constitute an armed group under international humanitarian law. It said Uganda's international partners should demand accountability and an independent investigation into whether the "massacre on Nov. 27 should be characterised as a crime against humanity.