Young veterans of West Africa's wars are being recruited to fight new conflicts across the region, according to a report by Human Rights Watch.
Fighters from the war in Sierra Leone moved on to other conflicts
The New York-based group says poverty is forcing thousands of young men and boys to become mercenaries.
It says fighters have been moving freely between conflicts in Liberia, Sierra Leone, Guinea and Ivory Coast.
The report warns war will continue to be seen as an economic opportunity unless alternatives are provided.
The study, entitled Youth, Poverty and Blood: The Lethal Legacy of West Africa's Regional Warriors, is based on interviews with about 60 veterans of West African conflicts.
Fighting to survive
It says many of the migrant fighters began their military careers as child soldiers, abducted to fight in wars, and many are guilty of war crimes and atrocities.
A man who began fighting with a rebel group in Sierra Leone is quoted as saying he was able to kill civilians in Liberia "because no-one knew me there - they weren't my people".
Economic hardship and the failure of disarmament efforts have led the men to fight for money and looting opportunities in fresh conflicts further afield, the report says.
A veteran of several wars in West Africa said he fought to support his parents.
"The commanders said we could pay ourselves, which meant looting," he told Human Rights Watch.
Hardship and brutality
The report says international efforts to disarm fighters and bring them back into society have only met with partial success.
"The fact that war has become the only economic route for thousands of youth points to serious failings by their countries' governments," said Peter Takirambudde, Human Rights Watch's Africa director.
Corruption among commanders and disarmament officials in Sierra Leone and Liberia is partly to blame, the report says.
New governments in these countries are said to have failed to correct many of the problems that had fuelled the original conflicts - inequity, warlord-ism and weak law and order.
More than two-thirds of the former combatants from Liberia interviewed by Human Rights Watch said they had received offers to fight in Guinea and the Ivory Coast within the last year.
The report says the migrant fighters were victims - as well as perpetrators - of human rights abuses, unable to escape lives of brutality, hardship and drug abuse.