Militias loyal to the former Ivory Coast president Laurent Gbagbo are recruiting child soldiers in Liberia to launch attacks similar to that which caused the death last week of 15 people, including seven UN peacekeepers.

Child soldiers as young as 14 are being groomed in training camps and used as scouts in increasingly deadly attacks in the volatile west of Ivory Coast, witnesses said. Human Rights Watch said that youths aged between 14 and 17 were being trained.

"They call us 'small boys unit,' and we are always safe when we go to the war zones in Ivory Coast. I don't know the total that we have killed," a child soldier told the campaigning group.

Ivory Coast's rugged western region is a stronghold of Gbagbo, whose refusal to leave power landed him in the international criminal court last May after five months of post-electoral conflict dislodged him.

But neighbouring Liberia has been reluctant to clamp down on mercenaries notorious for recruiting child soldiers, while high-profile members of the regime's inner circle live unhindered in upmarket villas in Ghana despite international arrest warrants.

"There are training camps in Liberia, you can walk there in 20 minutes. They send boys over several hours before attacks, then they join in later," said Traore Adama, an Ivorian soldier who fled to Abidjan after attacks in recent months. "Youths can make up to a third of the attacking groups. Some of them are older teenagers but others are waifs you'd never imagine carrying guns."

On Friday, UN "blue helmets", deployed to a suspected raid in a remote western village, were ambushed and killed by a large group of Liberian-based "militias or mercenaries," Paul Koffi Koffi, Ivory Coast's deputy defence minister, said. Eight civilians also died in the attack.

Liberia has since closed its 435-mile border with Ivory Coast, whose dense forests and maze of creeks make policing difficult. Hundreds began fleeing the mineral-rich region, where long-simmering ethnic tensions exploded in the massacres during last year's political conflict.

Alassane Ouattara, president of Ivory Coast since 2010, has struggled to unify a post-conflict army composed of quarrelling former rebels (who are also accused of war crimes) and government soldiers whom they once fought against.

One former rebel soldier, Aboubacar Souleymane, said" "It's very visible when an attack is imminent. Last week I told my superiors that Liberian mercenaries were planning an attack with the help of pro-Gbagbo supporters in the area. He said there was not enough evidence, so I fled the area. I wasn't prepared to stay there and get killed.

"There are too many cases of soldiers not being on their post when they should be, and disappearing from barracks at odd hours."

Officials say arms funding for the cross-border raids comes from Ghana, where several wanted Gbagbo insiders fled after the regime fell last year. Among them is Charles Blé Goudé, leader of the Young Patriots militia, whose armed supporters helped crush dissenters of the Gbagbo regime. Goudé, whom sources say is under investigation by the ICC, has said he is willing to face trial.

An African diplomat said cooperation was unlikely to be forthcoming from either of Ivory Coast's neighbours in the near future. "Liberia's armed forces' capability is limited by its small size and corruption because they're highly underpaid. But there's more than just a lack of capacity in both countries for failing to go after well-known criminals."