The Nigerian government has acknowledged it is getting technical and logistical support from what it calls foreign contractors in the fight against Boko Haram.
Nigerian government spokesman Mike Omeri said on Thursday that the government is not engaging in "any backchannel or unlawful recruitment".
In a statement emailed to The Associated Press, Omeri noted that soldiers from neighbouring countries, including Chad and Niger, were participating in operations against the group.
Omeri said other "individuals" from the region "are on the ground in a capacity limited to training or technical support".
But sources have told Al Jazeera that mercenaries from South Africa and other countries are actually playing a decisive fighting role on the frontlines.
A senior Nigerian non-commissioned officer, who is on active duty and was wounded by Boko Haram, said foreign mercenaries are doing the bulk of the fighting in towns the Nigerian military says it has recaptured from Boko Haram.
The officer said that the Nigerian military still suffers from a lack of equipment and low morale, and that without the mercenaries' help, these towns would still be held by Boko Haram fighters.
'Hundreds of mercenaries'
Security forces and diplomatic sources say Nigeria has brought in hundreds of foreign mercenaries in recent weeks from South Africa and other countries.
The reported presence of foreign fighters, equipped with heavy weaponry, attack helicopters and armoured vehicles, adds to the already broad array of forces, including soldiers from Chad, Niger, Cameroon and Benin, engaged in the battle to defeat Boko Haram.
Security operations have been stepped up as Nigeria prepares to hold general elections on March 28.
The polls were delayed by six weeks in order to buy time to recapture territory and restore stability in the country's conflict-ridden northeast.
In an interview with Voice of America late on Wednesday, Nigerian President Goodluck Jonathan said two companies were providing "trainers and technicians" to help Nigerian forces, without providing more details.
A West African security source and a South African defence source told the Reuters news agency that the foreign troops were linked to the bosses of former South African private military firm Executive Outcomes.
Executive Outcomes was best-known for its involvement in Angola's 1975-2002 civil war and against Revolutionary United Front rebels in an internal conflict in Sierra Leone in 1995.
The West African security source said several hundred foreigners were involved in running major offensive operations against Boko Haram, and were being paid around $400 a day in cash.
Their impact on the fighting so far could not be quantified, but the general run of the campaign has seen the tide turn somewhat against Boko Haram in recent weeks.
Separately, a South African defence contractor confirmed to Reuters that ex-Executive Outcomes leaders were involved in the deployment.
"It's an incoherent mix of people, helicopters and random kit from all sorts of different sources, but there is an element of internal cohesion from the Nigerian army," the diplomat said.
"It appears to be a desperate ploy to get some sort of tactical success up there in six weeks for the electoral boost," the diplomat added. The numbers of soldiers involved were in the "low hundreds", the diplomat added.
Source: Al Jazeera and agencies