Three female suicide bombers blew themselves up at the entrance to a camp for displaced people in north-east Nigeria, killing 28 people and wounding 82, and prompting a global humanitarian agency on Wednesday to call for better security to protect those displaced by Boko Haram violence.
The attack on Tuesday took place in the town of Mandarari, 25 kilometres from Maiduguri, the capital of Borno state, said Baba Kura, a member of a vigilante force set up to fight extremists.
"Three female bombers triggered their explosive outside of the IDP [internally displaced persons] camp ... killing 28 people and wounding 82 others," Mr Kura said.
The head of the Norwegian Refugee Council (NRC) in Nigeria, Ernest Mutanga said: "Camps sheltering innocent families fleeing war should be places of refuge. But instead they are turning into death traps."
"We need to see the Nigerian government stepping up to protect civilians in displacement camps. It's their primary responsibility."
The first assailant triggered panic when she blew herself up, Mr Kura said.
"People were trying to close their shops when two other female bombers triggered their explosives, causing most of the casualties," he said.
Ibrahim Liman, the head of a local anti-extremist militia force, confirmed the details of the attack, and said that more than 80 injured had been taken to Maiduguri hospital.
A source at the hospital said a "huge number" of patients had arrived.
North-east Nigeria is a hotbed of activity by the Boko Haram militant group, involving shootings, bombings and kidnappings.
The insurgents have stepped up their attacks on "soft" targets, Mr Mutanga said, indicating there had been 190 security incidents involving civilians in the region last month - more than the total combined for May and June.
There was also "an increase in the number of attacks on sites sheltering displaced civilians", he said.
The group launched an insurgency in 2009 that has killed about 20,000 people and displaced around 2.6 million others, creating one of the world's biggest humanitarian crises.
Nearly two million are suffering from acute malnutrition, according to UN figures.
After being elected to power in 2015, Nigerian president Muhammadu Buhari made tackling the insurgency a priority, winning back swathes of territory and declaring that Boko Haram is "technically defeated".
But there has been a surge in attacks this year, including an audacious assault on an oil exploration team that killed 69 people in Borno in July, raising fears that Boko Haram is regaining strength.
That attack prompted vice president Yemi Osinbajo - standing in for the president who is on indefinite sick leave - to tell the millitary to step up its response.
Mr Osinbajo issued "fresh directives … to immediately scale-up their efforts and activities in Borno state … to maintain a strong, effective control of the situation", his spokesman Laolu Akandehe said.
In August, 72 people were killed in north-eastern Nigeria, including 31 fishermen on islands in Lake Chad, according to an unofficial toll compiled from news reports.