There are widespread reports that Muammar Gaddafi has unleashed numerous foreign mercenaries on his people, in a desperate gamble to crush dissent and quell the current uprising.

Their origins vary according to speculation: Chad, the Democratic Republic of Congo, Niger, Mali, Sudan and possibly even Asia and eastern Europe.

The claims are hard to pin down but persistent. Ali al-Essawi, the Libyan ambassador to India, who resigned in the wake of the crackdown, told Reuters on Tuesday: "They are from Africa, and speak French and other languages."

He said their presence had prompted some army troops to switch sides to the opposition. "They are Libyans and they cannot see foreigners killing Libyans so they moved beside the people."

In a separate interview, Essawi told al-Jazeera: "People say they are black Africans and they don't speak Arabic. They are doing terrible things, going to houses and killing women and children."

Witness accounts seem to bear out the claims. One resident of Tripoli was quoted by Reuters: "Gaddafi obviously does not have any limits. We knew he was crazy, but it's still a terrible shock to see him turning mercenaries on his own people and just mowing down unarmed demonstrators."

Saddam, a 21-year-old university student in Bayda, claimed mercenaries had killed 150 people in two days. "The police opened fire at us," he said. "My friend Khaled was the first martyr to fall and seven others died with him.

"The next day, we were shocked to see mercenaries from Chad, Tunisia, Morocco speaking French attacking us ... We captured some of the mercenaries and they said they were given orders by Gaddafi to eliminate the protesters."

Amid the chaos gripping Libya, the volume of foreign mercenaries and much else remains confused. Some believe they could be veterans of civil wars in the Sahel and west Africa.

Ibrahim Jibreel, a Libyan political activist, told al-Jazeera that some had been in the country for months, based in training camps in the south, as if in anticipation of such an uprising. Others had been flown in at short notice, he said.

Some reports suggest white mercenaries have also been spotted fighting on Gaddafi's behalf. White South Africans who left the national army after the end of racial apartheid have been in demand for their expertise in various war zones, including Afghanistan and Iraq, but there is currently no evidence that any have joined the conflict in Libya.

Experts suggest that Gaddafi has plenty of options in the region. "He has traditionally had a network of skilled soldiers from all over west Africa," said Adam Roberts, author of The Wonga Coup, the story of a failed attempt by Simon Mann and other mercenaries to overthrow the president of Equatorial Guinea in 2004. "There are lots of Africans, particularly from west Africa or Sudan, who go to Libya because it's wealthier."

Mercenaries remain a potent weapon against civilian populations, despite the African Union's 1977 Convention for the Elimination of Mercenarism in Africa. Liberian civil war veterans have been hired by Ivory Coast president Laurent Gbagbo to terrorise protesters following his widely acknowledged election defeat.

Roberts added: "Gaddafi and other dictators tend to surround themselves with fighters who will be loyal to them rather than to a local faction. Foreign mercenaries are likely to be less squeamish about shooting at local people.

"They are likely to better trained – a small unit that can be relied upon. They might also have experience of fighting battles and therefore be more capable if push comes to shove."

The view was echoed by Reed Brody of Human Rights Watch. "It's hard to get your own people to shoot your own people," he said. "In this kind of situation, you can see why mercenaries would be an advantage because it's easier to get foreigners to shoot at Libyans than to get Libyans to shoot at Libyans."

Gaddafi can offer mercenaries what they want more than anything: money. Sabelo Gumedze, a senior researcher at the Institute for Security Studies (ISS) in South Africa, said: "Mercenaries are purely driven by profit. As long as they make money, they're going to do it, and leaders like Gaddafi have money at their disposal."

There is a constant supply of willing recruits, he added. "In Africa the process of demobilising rebels is poor. The only thing they know is how to fight. If someone can turn the barrel of a gun into profit, they jump at it. They have few other employment opportunities."

José Gómez del Prado, chair of the working group on the use of mercenaries at the UN human rights council, said: "You can find, particularly in Africa, many people who've been in wars for many years. They don't know anything else. They are cheap labour, ready to take the job for little money. They are trained killers."

Del Prado said he has heard the reports of mercenaries in Libya from a number of sources and is "very worried".

But some analysts urged against jumping to conclusions in Libya, noting that the country has a significant black population who may simply be serving in the regular army and could be mistaken for mercenaries. These include Chadians who sided with Gaddafi in his past conflicts with Chad and were rewarded with houses, jobs and Libyan citizenship.

The UN refugee agency (UNHCR) said yesterday it had received "alarming reports" that Libyans were turning on African refugees whom they accused of being mercenaries.

Issaka Souare, a senior researcher at the ISS's Africa conflict prevention programme, said: "In the south of Libya you do have people of sub-Sarahan origin, including Hausa speakers. Some might have integrated into the Libyan army and these would probably be among the first to be deployed. It will then be easy for people to say they are foreign mercenaries.

"People started talking about this issue on the third day, but I think Gaddafi should have had sufficient resources to deal with the protests before resorting to mercenaries. How long would it take Gaddafi to get mercenaries together and deploy them? Maybe a week. So I see it as unlikely at this stage, but it could happen if army defections continue."