"The use of lethal force by law enforcement officials from September 26-29 in Myanmar was inconsistent with the fundamental principles reflected in the basic international norms deriving from international customary law," the UN special rapporteur added.
Detainees remain subject to cruel, inhuman and degrading treatment and torture with political activists and human rights defenders targeted, Pinheiro said.
More than 600 people have been detained and 74 listed as missing in the wake of the crackdown, Pinheiro said, adding that Myanmar's government had not taken serious steps to respect human rights.
The UN expert visited Myanmar in November, but was unable to meet Aung San Suu Kyi, the jailed pro-democracy leader.
He said he was "reassured by the authorities that this option will remain on the agenda of his follow-up missions."
The crackdown in September was sparked by protests against a steep rise in fuel prices in August, but which rapidly escalated into demonstrations against the military regime which has ruled Myanmar for decades.