South Africa was one of the first countries in the world to ban mercenaries but remains a major supplier of military instructors, some of them from the time of the brutal apartheid regime.

Former Koevoet officer Leon Lotz, who was relatively well-known in South Africa, was one of them.

On Wednesday, he was killed in a friendly fire incident in northeast Nigeria, where a regional force has been battling Boko Haram insurgents.

The Koevoet was a South African special forces unit tasked in the 1980s with putting down the Southwest African People's Organisation (SWAPO) liberation movement in occupied Namibia.

Lotz was 59 when he was killed, according to Netwerk24, one of the few websites written in Afrikaans, the language of the white South African minority.

His wife, Almari, was quoted as telling the site that her husband "was with some of his brothers-in-arms who have walked a path with him for many years".

The family has since then refused to speak publicly.

The silence is all the more understandable given that providing unauthorised military services abroad is illegal in South Africa -- and that Lotz may have been paid by the Nigerian government.

Nigeria has refused to comment officially on the reports that foreign mercenaries are involved in the counter-insurgency but President Goodluck Jonathan has hinted at their presence.

In an interview with Voice of America published on Wednesday, Jonathan said "foreign technicians" were present in northeast Nigeria to assist the military but did not elaborate.

- 100+ South Africans? -

Lotz is far from being an isolated case, even if the exact number of white South African former soldiers popping up elsewhere in the world remains unclear.

"The short answer is we don't know (how many there are), they don't talk," the managing editor of the African Defence Review, John Stupart, told AFP.

But he estimated that there may be more than 100 South Africans currently in Nigeria, with reports that Georgian and Ukrainian mercenaries are also on the ground.

"'Mercenaries' is the common name, I call them 'private military contractors'. It's the most neutral term," said Stupart.

"The skills of these South African veterans are actually very, very good.

"Their reputation as counter-insurgency specialists... during the bush war during the apartheid years... makes them highly valuable.

"To be honest, I don't think they are being used anymore in combat... simply because those guys are pushing 60 or 70."

The United Nations has said that since apartheid ended in 1994, "many South Africans with extensive military skills and expertise have been unwilling or unable to find employment in South Africa.

"As a result, they have offered their services abroad and many have been employed by international private military and security companies," the Human Rights Council said in a 2011 report.

"Some have become involved in mercenary activities."

Others have also joined private US military companies such as DynCorp, which was involved in African conflicts, according to the most recent Africa in Fact, looking at the continent's armies.

- Political window-dressing? -

Between 1995 and 2000, the number of men in the South African National Defence Force dropped significantly from 120,000 men to 82,000, the publication said.

An anti-mercenary law -- the Foreign Military Assistance Act -- was swiftly implemented in 1998 after the end of apartheid.

The law notably saw the dismantling of the notorious "security and military consultancy" Executive Outcomes, which was founded in South Africa by a former army officer in 1989.

It worked for the government of Sierra Leone during the brutal civil war and in Angola.

Pretoria moved to strengthen the law following a failed coup in Equatorial Guinea in 2004, which implicated a former British special forces soldier Simon Mann and more than 60 others.

But according to Stupart, the law is political window-dressing.

"(It) has never been really enforced and to be honest, I don't think it is enforceable because it's very blurred," he said

Several thousand South Africans worked in Iraq in 2011 and 35 were killed between 2004 and 2010, according to the UN report.

A French-born South African, Francois Rouget, was one of the first people to face the anti-mercenary law.

In 2003, he was convicted of having recruited former members of the apartheid-era South African army in 2002 for the Ivory Coast government of Laurent Gbagbo.