The suspected presence of mercenaries was first reported by Reuters. The report confirmed that hundreds of mercenary fighters from South Africa and countries in the former Soviet Union were already on the ground in northeastern Nigeria, including in the city of Maiduguri, the capital of Borno state, which is at the heart of Boko Haram's bloody insurgency.
Nigerian officials have said publicly that foreign contractors are simply helping with the training of Nigerian forces, but anonymous sources have confirmed to various outlets that the mercenaries are indeed playing a far more direct role in the fight.
"They are playing a very important role," a senior Nigerian official told the New York Times, describing how mercenaries have helped roll back Boko Haram's advance. "They are in the vanguard in the liberation of some of the communities. They came in with much more sophisticated equipment than the military. Thanks to their involvement the tide is turning. I believe because of them we will witness a seismic shift."
At least one South African private contractor, 59-year-old Leon Lotz, has been reported killed in clashes with Boko Haram.
Boko Haram, an Islamist terrorist group that's now a self-proclaimed affiliate of the Islamic State, has carved a gruesome trail through northern Nigeria and carried out attacks also in neighboring Cameroon and Niger. They have abducted hundreds of people and slaughtered thousands -- more than 4,700 people just last year -- and continued their operations despite a heavy-handed Nigerian military response.
The threat posed by Boko Haram even prompted the Nigerian government to postpone national elections, originally scheduled for last month, to the end of March.
A coalition of 8,700 troops from Chad, Niger, Cameroon and Benin have also joined the battle. A joint offensive into Nigerian territory by forces from Niger and Chad recently managed to wrest a number of towns away from Boko Haram's clutches.
The actions of the mercenaries are a bit more murky. One report suggests "white" foreign soldiers are manning Nigerian mobile-rocket launchers and even piloting Nigerian jets as part of the counteroffensive against Boko Haram.
The mercenaries carry with them a long, tainted history. In 2006, South Africa made it a crime for citizens to pursue work as security staff overseas without permission from the government. The South African mercenaries tend to be white, and many were former soldiers or special forces in the apartheid-era regime. Over the past few decades, they have popped up in various corners of Africa, often as agents for coup-plotters or on one side or the other of a civil war.
Critics say their work, carried out for private gain, has in the past undermined democratic governments -- they represent to some a kind of neo-colonialist scourge.
South Africa's current defense minister has declared that any South Africans known to be fighting in Nigeria will be arrested and prosecuted upon their return home.