More than 50 people have been killed in clashes between ethnic groups in eastern Democratic Republic of the Congo, three local aid workers said on Sunday, the largest death toll in fighting between the two groups for months.

Violence has raged across much of Congo this year, killing hundreds and displacing millions amid a political crisis caused by President Joseph Kabila’s refusal to step down when his constitutional mandate expired in December.

On Friday, a group of Twa pygmies attacked the Luba, a Bantu ethnic group, near Kalemi, in Tanganyika province, the aid workers said.

The pygmies, who live in Central Africa’s Great Lakes region, have fought the Luba for decades over what activists say are inequalities between Bantu villagers and the Twa hunter gatherers, are excluded from access to land and basic services.

“According to information gathered, there were about 50 deaths in the attack on the group,” Ilunga Musafiri, president of the local NGO Inter-church Council said.

A local activist said most of the casualties were Lubas.

U.N. officials are concerned that violence is spreading out of control in Congo with Kabila’s complicity. Critics have said he may use the insecurity as an excuse to delay elections.

A U.N. report last week found 251 killings took place in three months this year in the diamond-rich central Kasai region in violence that has involved child soldiers and witchcraft.

The number of people displaced in Congo has more than doubled to 3.7 million since August 2016, the U.N. said in May.