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.
The UN helicopter accidentally landed in al-Shabab territory on Wednesday, with 6 of 9 passengers taken hostage.