It was one of the most notorious private armies of the late apartheid era: a company of South African mercenaries who fought brutal wars to crush African rebels on behalf of governments and private corporations.

The company, Executive Outcomes, was disbanded in 1998 after criticism from Nelson Mandela's post-apartheid government, but some of its former members have now surfaced in northern Nigeria, where they are reported to be among hundreds of mercenaries fighting Boko Haram for $400 (U.S.) in cash per day.

South Africa, which has outlawed mercenaries, has threatened to arrest any of its citizens who travel to Nigeria to engage in combat, but the threat has been ignored by the South Africans who are battling the Islamist radical militia for cash payments. Foreign mercenaries and Chadian troops have played a crucial role in helping Nigeria recapture dozens of towns from Boko Haram in recent weeks.

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With a national election approaching, the Nigerian government is under political pressure to defeat Boko Haram before the vote on March 28. Its use of mercenaries is no secret in northern Nigeria, where foreign military contractors from South Africa and the former Soviet Union are often seen in the streets.

Nigerian politicians have told The Globe and Mail that they obtained illicit, black-market military equipment to fight Boko Haram because they were unable to get support from Western governments.

But the latest reports from South African media and the Reuters news agency suggest that the use of mercenaries is far more extensive than previously known. Several hundred mercenaries – including pilots of helicopters and fighter jets – are believed to be involved in the battle against the Boko Haram insurgents. At least one has already been killed.

Leon Lotz, a former South African soldier who became a private security contractor, was killed on Monday in a friendly fire incident when a Nigerian tank opened fire on his convoy near Boko Haram territory, mistakenly believing it was a rebel convoy, according to South African newspapers and websites.

The proliferation of security forces fighting Boko Haram – including Nigerian soldiers and police, Chadian soldiers and foreign mercenaries – could greatly heighten the risk of confusion and friendly fire blunders similar to the one that killed Mr. Lotz.

One South African news site, Daily Maverick, reported that Mr. Lotz was working for a private security company that was owned by a former member of Executive Outcomes. A Namibian contractor and five Nigerian soldiers were also reportedly killed in the attack.

Nigerian media have recently published several photos of soldiers in armoured vehicles in northern Nigeria who appear to be white foreigners. They were identified as South African.

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In January, media reports in South Africa said at least 100 former South African soldiers had joined the military campaign against Boko Haram. At the time, South African Defence Minister Nosiviwe Mapisa-Nqakula said the former soldiers were mercenaries who should be arrested and prosecuted when they return.

"There are consequences for going out of the country and providing any form of military assistance as a mercenary," she told reporters. Prosecutors should "secure a conviction" so that it "sends a message" to any South African mercenaries, she said.

South Africa passed its law against mercenaries in 1998, shortly before Executive Outcomes was disbanded, although the company denied that its demise was a result of the law. Executive Outcomes became infamous for fighting rebels in Angola and Sierra Leone in the 1990s, including battles to dislodge rebels from oil facilities and diamond mines.

A report on Thursday by the Reuters news agency, quoting West African and South African security sources, said the mercenaries in northern Nigeria were linked to the former bosses of Executive Outcomes. It said Nigeria was paying $400 a day in cash to several hundred foreign mercenaries from South Africa and the former Soviet Union.

One diplomat told Reuters that the hiring of foreign mercenaries "appears to be a desperate ploy to get some sort of tactical success up there in six weeks for the electoral boost."

The election was originally scheduled for Feb. 14 but was delayed by six weeks after the Nigerian military announced that it planned a major offensive against Boko Haram, beginning on the election date.