Ivory Coast President Alassane Ouattara

ACCRA, Ghana(AP) -- Armed groups in Liberia who supported Ivory Coast's former president have killed at least 40 civilians in cross-border raids into Ivory Coast since July and are recruiting children as young as 14 into their ranks, a human rights group said Wednesday.

Human Rights Watch says the armed men, most of whom fought for Ivory Coast's former president and flooded over the border to Liberia following his arrest, carried out at least four attacks targeting ethnic groups who support Ivory Coast's current president, Alassane Ouattara.

Ivory Coast was brought to the brink of civil war when former President Laurent Gbagbo refused to cede power to Ouattara in a 2010 election. The U.N. estimates at least 3,000 people were killed in the six months of violence that followed. Gbagbo was arrested with the help of U.N. and French forces in April 2011, and is now facing charges of war crimes at The Hague.

Both sides handed out weapons and recruited young men to fight during the conflict. Several thousand Liberian mercenaries joined the fight, the vast majority for Gbagbo's side, Human Rights Watch says. Following Gbagbo's arrest, many of the mercenaries and militiamen who fought for him fled across the porous border into Liberia's forests, or clandestinely, into its refugee camps.

The New York-based rights group says the Liberian government has failed to respond to the presence of armed groups on the border or to the recruitment of child soldiers.

``Rather than uphold its responsibility to prosecute or extradite those involved in international crimes, Liberian authorities have stood by as many of these same people recruit child soldiers and carry out deadly cross-border attacks,'' said Matt Wells, West Africa researcher at Human Rights Watch.

There was no immediate reaction to the report by the Liberian government, though Ivory Coast deputy defense minister Paul Koffi Koffi said the Ivorian and Liberian authorities are collaborating to prevent further attacks.

``We're working with the Liberians and we have reinforced patrols along the border,'' Koffi Koffi said. He said there was a joint military program in place, but that it was secretive and he could not provide details.

Human Rights Watch said it had documented armed groups recruiting Liberian children and residents of several Liberian border towns also described seeing children at a training camp for fighters. A 17-year-old boy told the group he led a unit that included other children and that they had participated in cross-border attacks.

The rights group says the government is also responsible for releasing ``war criminals'' from prison. In April, Liberian authorities released Isaac Chegbo on bail, a mercenary better known as ``Bob Marley'' for his long dreadlocks. Chegbo is accused of leading massacres in Ivory Coast last year that left more than 120 people dead.