At least four people were killed in May when Iranian security forces launched a violent crackdown on protests over the rising cost of living, firing live ammunition and birdshot, Amnesty International said Wednesday.

It is the first time a top rights group has given a toll based on analysis of the evidence.

In a report, Amnesty said the Iranian authorities needed to be held accountable for the "torrent of violence" unleashed against the protesters in the southwest of the country.

"The authorities' militarized response laid bare once again their utter disregard for the sanctity of human life and international legal standards on the use of force and firearms," said Diana Eltahawy, Amnesty International's Deputy Director for the Middle East and North Africa.

The protests over rising food prices erupted in southwestern Khuzestan province in early May, then spread to neighboring Chaharmahal and Bakhtiari provinces.

Amnesty said video and audio evidence indicated that the security forces "fired weapons loaded with live ammunition and birdshot on multiple occasions during the protests."

Birdshot – where small spheres are crammed inside a shell and then scatter outward once fired – "is inherently indiscriminate and designed to cause a level of widespread harm," Amnesty said.

Three people – Behrouz Eslami, Jamshid Mokhtari and Saadat Hadipour – were killed in Chaharmahal and Bakhtiari provinces in connection with the protests between May 14 and 17, it said.

Meanwhile, on May 15 a member of parliament also reported the death of an unnamed person in Khuzestan province in connection with the demonstrations.

Protests also took place later in the month after the deadly collapse of a building in the Khuzestan city of Abadan, where authorities fired shots "at crowds of grieving protesters," the report said.

Amnesty said there were "cycles of protest bloodshed" in Iran with an "increasing militarization of the policing of protests."

The group has already said at least 321 men, women and children were killed by Iran's security forces during a crackdown on nationwide protests in November 2019 following a surprise fuel price rise.

"Iran's security forces will continue to feel emboldened to kill and injure protesters if they are not held accountable," said Eltahawy, reaffirming Amnesty's demand for the United Nations to establish an independent mechanism to investigate such crimes in Iran.