Soldiers targeting the Kamwina Nsapu group in central Democratic Republic of Congo killed at least 101 people over five days in February, the UN said.
At least 39 women were among the victims killed amid the violence between February 9 and 13, the UN's human rights spokeswoman Liz Throssell said on Thursday, citing sources in the country.
The soldiers fired indiscriminately with machine guns when they saw the fighters, who were armed mainly with machetes and spears, she said.
"We are deeply concerned at the reported high number of deaths, which, if confirmed, would suggest excessive and disproportionate use of force by the soldiers," she said.
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The violence is also part of a broader surge in instability across Congo in recent weeks after President Joseph Kabila refused to step down when his constitutional mandate expired in December.
At least 11 people were killed on Monday in clashes between the army and a group loyal to a traditional chief slain in fighting with police last year, said local activist Jean Rene Tshimanga, president of the Civil Society of Kasai-Central province.
"This morning, we learned again that [the group] attacked the men in uniform [who] repelled them," Tshimanga told Reuters news agency. He did not know how many of the dead were fighters and how many army soldiers.
Neither provincial nor military officials could be immediately reached for comment.
Similar clashes have uprooted tens of thousands.
Analysts say violence in Congo, a tinderbox of conflicts linked to land, ethnicity and mineral resources, has been exacerbated by Kabila's failure to step down.
On Saturday, the Congo's UN peacekeeping mission said Kamwina Nsapu had committed violent atrocities and used child soldiers, and it also criticised the army for what it said was a disproportionate use of force against the militia fighters, who are typically only lightly armed.
Source: Reuters news agency