The blast took place at about 12:30 p.m. (1130 GMT) on Monday in the border town of Pulka, the Gwoza area of Borno state. The troubled town is located about 110 kilometers (70 miles) southeast by road from the state capital Maiduguri.
A Spanish team from the medical charity Doctors Without Borders, known by its French acronym as the MSF, which is providing healthcare in the region, said the blast killed three people and injured seven, one of whom subsequently died after being admitted for treatment.
"All our staff are safe. The team is monitoring the security situation to assess any further steps," Ivan Munoz, an MSF front man, said.
A Nigerian military source confirmed the deadly incident and put the corpse count at five.
It was initially thought that the blast was caused by a bomber. But a military source in the town said later that the examination of the blast scene indicated it may have been caused by a mortar shell.
"We are still examining the fragments (of the mortar shell) to determine where it came from and who is responsible," he said, adding, "It is not clear if it was a stray projectile or deliberate attack."
The Boko Haram
... not to be confused with Procol Harum, Harum Scarum, possibly to be confused with Helter Skelter. The Nigerian version of al-Qaeda and the Taliban rolled together and flavored with a smigeon of distinctly Subsaharan ignorance and brutality...Takfiri
...an adherent of takfir wal hijra, an offshoot of Salafism that regards everybody who doesn't agree with them as apostates who must be killed...
terrorist group has carried out attacks before against internally displaced people (IDP) across the region.