Four people were killed in a blast at a camp for people displaced by the Boko Haram conflict in northeast Nigeria, Doctors Without Borders (MSF) said on Tuesday.
A Nigerian military source confirmed the incident and put the death toll at five.
The explosion happened at about 12:30 pm (1130 GMT) on Monday in the border town of Pulka, in the Gwoza area of Borno state, where a Spanish team from the medical charity is providing healthcare.
It was initially thought the blast was caused by a suicide bomber.
But a military source in the town said later that examination of the blast scene indicated it may have been caused by a mortar shell.