Updated

The United States and others are urging Ugandan authorities to fully investigate a military attack on a tribal king's palace two years ago that killed over 100 civilians and was widely condemned.

At least 15 children were among the victims of the November 2016 assault targeting a kingdom known as Rwenzururu, whose leader for years has been politically opposed to Uganda's longtime President Yoweri Museveni, according to Human Rights Watch.

The killings followed clashes between police and followers of the tribal king, who later was arrested and charged along with hundreds of supporters. Many remain in detention.

The U.S. and European Union are demanding justice for victims of the attack whose perpetrators in the police and the military have never been punished. "How much longer will they wait?" the U.S. statement tweeted Tuesday said. The EU urged a transparent investigation.

Uganda insists the attack targeted armed militiamen and that the matter is before the criminal justice system. No arrests of military actors have been announced, and the operation's commander, Maj. Gen. Peter Elwelu, was later promoted to become the commander of Uganda's land forces.

The government has often reacted angrily to allegations of a massacre at the hands of the security forces, denying that children were killed.

"Ugandan officials won't even ask why overwhelming lethal force was used that day and why children died, which shows a terrifying disdain for human life," Maria Burnett, East Africa director at Human Rights Watch, said in a statement.

Amnesty International has said many victims appear to have been shot dead, with their bodies dumped.

Museveni, who took power by force over three decades ago, has struggled to win the support of the Rwenzururu people, who complain of marginalization by the central government 340 kilometers (211 miles) away in the capital, Kampala.

Museveni lost the vote in Kasese, seat of the Rwenzururu kingdom, in the presidential election in 2016. Clashes erupted a month later between security forces and militiamen loyal to the Rwenzururu king. Among the king's supporters were separatists who hoped to create a new republic.

There have been frequent land disputes in the area, with many accusing the government of sponsoring land grabs.

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