By Ed Cropley and David Lewis

JOHANNESBURG/DAKAR (Reuters) - Nigeria has brought in hundreds of mercenaries from South Africa and the former Soviet Union to give its offensive against Boko Haram a shot in the arm before a March 28 election, according to regional security, defense and diplomatic sources.

Rumors about the use of foreign "soldiers of fortune" against the Islamist militant group gained substance this month when pictures surfaced on Twitter showing armored vehicles rumbling along a street in what was said to be Maiduguri, the regional capital of Nigeria's Boko Haram-hit northeast.

In one photo that appeared on Twitter on March 6, a white man in a khaki tee-shirt and body armor is shown beside a heavy-caliber machine gun on top of one of the sand-colored vehicles as the column drives through the streets at dusk.

A Reuters reporter with knowledge of Maiduguri was able to verify the location of the photo as the Bama road, leading southeast out of the city, near the University of Maiduguri.

Election campaign posters of Borno state governor Kashim Shettima hanging from street lights indicate it was taken recently. The lights, notable for their ornate ironwork, were only installed last year.

In confirming the presence of hundreds of foreign military contractors on the ground, including recently in the city of Maiduguri, security and diplomatic sources put the total much higher than the hundred or so previously reported.

Nigerian government spokesman Mike Omeri declined to comment, referring questions to military spokesman Chris Olukolade, who also declined to respond to multiple requests for comment.

In an interview with Voice of America late on Wednesday, President Goodluck Jonathan said two companies were providing "trainers and technicians" to help Nigerian forces. He did not name the firms, or the nationalities, or give numbers.

But a West African security source and a South African defense source said the foreign troops were linked to the bosses of former South African private military firm Executive Outcomes.

Story continues

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. It disbanded in 1998, under pressure from the post-apartheid government in Pretoria to curtail mercenary activities.

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 defense contractor confirmed to Reuters that ex-Executive Outcomes leaders were involved in the deployment, which comes after the six-week postponement of elections in mid-February due to the threat from Boko Haram.

One Abuja-based diplomat said the South Africans were backed by soldiers and hardware from the former Soviet Union in an alliance against Boko Haram, which has killed thousands of people in its six-year campaign to establish an Islamic caliphate in northern Nigeria.

"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.

"NO BUSINESS TO BE THERE"

John Stupart, editor of African Defence Review, identified the troop carriers as Reva III, manufactured by a Pretoria-based company called Integrated Convoy Protection.

After reports of South African military trainers first surfaced in the Afrikaans-language Beeld newspaper in January, Defence Minister Nosiviwe Mapise-Nqakula made clear her displeasure, saying any deployment would be illegal under 1998 anti-mercenary laws.

"They are mercenaries, whether they are training, skilling the Nigerian defense force, or scouting for them. The point is they have no business to be there," she was quoted as saying in domestic media this month.

South Africa bans its nationals from participating directly in hostilities for private gain. Georgia, seen as a major source of mercenaries, has laws before parliament criminalizing participation in a broad range of foreign military activities.

Reuters was unable to reach the former bosses of Executive Outcomes through military contacts in South Africa.

The appearance of foreign private soldiers comes four months after Nigeria’s ambassador to the United States said Washington was not helping the struggle against Boko Haram, and had failed to share intelligence and sell Nigeria the weapons it needed.

The presence of mercenaries from South Africa and the former Soviet Union adds to the broad array of forces lining up against Boko Haram, which has emerged in the last few years as sub-Saharan Africa's biggest security threat.

Chad, Niger, Cameroon and Benin have committed troops to an 8,700-strong regional force. This week, Chad and Niger launched a joint military offensive deep into Nigerian territory.

U.S. and European special forces have just completed three weeks of war games with regional counterparts near Lake Chad, one of boundaries of a Boko Haram sphere of influence thought at one time to be the size of Belgium.

(Story corrects name of magazine in paragraph 17)

(Additional reporting by Tim Cocks, Isaac Abrak, Julia Payne and Lanre Ola; Editing by Giles Elgood)