Amnesty International says an attack by the Boko Haram terrorist group on Nigeria’s northeastern town of Rann has left at least 60 people dead, denouncing Nigerian authorities for their failure to protect civilians.

The UK-based rights group said in a statement on Friday that the attack had been carried out on Rann in Borno State on Monday and that it “was the deadliest yet by Boko Haram.”

“This attack on civilians who have already been displaced by the bloody conflict may amount to [a] possible war crime, and those responsible must be brought to justice,” said Osai Ojigho, Amnesty’s Nigeria director.

She also condemned as “unacceptable” the Nigerian authorities’ failure to protect people.

“Witnesses told us that Nigerian soldiers [in the area] abandoned their posts the day before the attack, demonstrating the authorities’ utter failure to protect civilians,” she said.

Amnesty also analyzed satellite images that showed hundreds of structures burned in the east, south, and southeast of Rann.

“Using satellite imagery, we have also been able to confirm the mass burning of structures as Boko Haram unleashed a massive assault on Rann, most of which is now destroyed,” said Ojigho.

Earlier, on January 14, the Takfiri militants had attacked a military base and set fire to over 100 structures in Rann, leaving seven people dead.

The town is currently sheltering some 35,000 internally displaced persons (IDPs), according to the International Organization for Migration.

In January, the United Nations warned about growing violence by Boko Haram militants in northeastern Nigeria, saying more than 30,000 people — mostly women and children — have fled their homes in the conflict-ridden area over the past weeks.

Boko Haram’s nine-year militancy is estimated to have killed more than 27,000 people and forced 1.8 million others to flee their homes.

In 2015, Boko Haram pledged allegiance to the Takfiri Daesh terrorist group.