Bamako has cut ties with Kyiv, accusing it of complicity in an attack that killed Malian soldiers and Wagner troops.
The attack on June 11 was one of the deadliest suffered by the West African nation’s army.
The UN official said he was ‘appalled’ by the killings in central Mali.
The army’s fight against rebels has been criticised as heavy-handed, with the HRW report being the latest criticism.