Geologists were surveying the Lake Chad basin for crude oil when they were set upon by militants.
Boko Haram ambush in Nigeria claims more than 50 lives
More than 50 people have died after an ambush by Boko Haram, an Islamist extremist group, in northeastern Nigeria, according to Agence France-Presse.
The bodies of 18 soldiers and 30 others were brought back to Maidguri after the attack, which took place on Tuesday, in an area near Lake Chad.
The government has suspended oil prospecting in the area after the militants targeted geological surveyors sent out to explore the site in Borno state.
Nigeria's military said on Wednesday that 10 people had died in the attack by the jihadists.
However, this has been disputed by sources dealing with the aftermath, who say the number of dead is much higher and is still rising.
One source told AFP on Thursday: "The death toll keeps mounting. Now we have more than 50... and more bodies are coming in.
"It's clear that the attack wasn't for abduction. They (Boko Haram) attacked just to kill."
The military exercises strict control over the area of Borno, meaning details of what exactly happened have been slow to emerge.
Emmanuel Ibe Kachikwu, Nigeria's junior oil minister, said in a statement the attack was "unfortunate" but did not say how many had died.
The ambush came just days after the head of the Nigerian army handed down an order to troops capture Boko Haram's leader, Abubakar Shekau, dead or alive within 40 days.
Tukur Buratai, chief of army staff, also called upon members of the public to help in securing Shekau's capture.
Boko Haram's insurgency, which began in 2009, has claimed 20,000 lives and left 2.3 million people displaced.
The latest attack, the deadliest from the jihadists in recent months, contradicts government claims that Boko Haram is close to being eradicated.