At least four people have been killed and several others injured in an explosion at a camp for people displaced by militancy in northeastern Nigeria.

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 spokesman, said.

A Nigerian military source confirmed the deadly incident and put the death toll 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 Takfiri terrorist group has carried out attacks before against internally displaced people (IDP) across the region.

In September last year, at least seven people were killed when Boko Haram terrorists fired an RPG at a camp at Ngala in Borno state near the border with Cameroon.

More than 20,000 have been killed and over 2.5 million have been displaced in almost eight years of insurgency by Boko Haram in northern Nigeria and the neighboring countries of Chad, Niger and Cameroon. The Nigerian authorities claim the group has “technically” been defeated as a result of a massive operation that began in early 2015. However, civilians and security forces in northern Nigeria are still targeted in sporadic attacks.