The South African government is concerned its nationals may be working as mercenaries in Nigeria in the war against the rebel group Boko Haram.
A Nigerian government spokesman, Mike Omeri, told the BBC that foreigners were just training troops in the use of new weapons.
Nigeria has recently bought helicopter gunships and tanks from South Africa and former Soviet Union countries.
But the South African government says the men may be involved in fighting.
It is against the law for South Africans to fight overseas for private gain. South Africa’s Defence Minister Nosiviwe Mapisa-Nqakula told the BBC that anyone suspected of fighting in Nigeria would be investigated by South African police.
No serving members of South Africa’s armed forces were in Nigeria, the minister added.
The presence of South African mercenaries was first reported by a South African newspaper in January, and speculation intensified after a photograph circulated on Twitter last week showing a white man in a khaki T-shirt and body armour, next to a heavy-calibre machine gun.
The location of the photo was later identified as Maiduguri in north-east Nigeria, close to the fighting with Boko Haram.
On Thursday, South African websites reported that a mercenary from the country had been killed in an incident of so-called friendly-fire in Nigeria. Two anonymous sources speaking to the French news agency AFP confirmed the incident.
The South African government has acknowledged the death of one of its citizens in Nigeria, the New York Times reports.
“We are disturbed by the death of this one person,” defence spokeswoman Joy Peters told the paper.
“Unfortunately, they went to Nigeria in their own personal capacity. We’d like to advise that this would serve as a warning to others who are considering engaging in such activities to really think twice and consider the repercussions.”
Alongside South Africans, mercenaries from former Soviet Union countries are also reported to be taking part in the conflict.
In addition, regular soldiers from Chad, Niger and Cameroon have been working with the Nigerian military to recapture towns and villages in north-east Nigeria which have fallen under the control of the Islamist rebels.
SOURCE: BBC News