A rights group says militants in the West African state of Liberia are using children to conduct cross-border attacks on villages in Ivory Coast.Human Rights Watch says the attacks have killed at least 40 people since July 2011.Hundreds of Liberian mercenaries fought on behalf of deposed former Ivorian President Laurent Gbagbo in last year's political unrest.Along with Ivorian militants, many crossed the border back into Liberia.The New York-based group says in a report that militants are recruiting children as young as 14 to attack Ivorians who are aligned to President Alassane Ouattara and live close to the border. MercenariesLast July, Mr Gbagbo's refusal to concede power to Mr Ouattara after a December 2010 presidential election escalated into violence.Mr Gbagbo was eventually captured and handed to the International Criminal Court (ICC) to be tried on charges of crimes against humanity.Hundreds of Liberian mercenaries, as well as Ivorian militants fighting on Mr Gbagbo's behalf, are now living in Liberia and have not been prosecuted by authorities in the capital Monrovia, according to Human Rights Watch.The report is based on field work in Liberian towns bordering Ivory Coast where HRW conducted interviews with 21 Ivorian and Liberian former mercenaries.One 17-year-old boy interviewed said that he led a unit that included other children to take part in cross-border attacks.Other people said they saw children aged between 14 and 17 at training camps.The report echoes concerns expressed by the United Nations that the Liberian government had not taken adequate measures against Liberian mercenaries and Ivorian militants accused of war crimes."We are calling on the Liberian government to uphold its responsibilities under Liberian and international law to investigate and prosecute those who committed war crimes in Ivory Coast and those involved in more recent cross-border attacks," HRW West Africa researcher Matt Wells told the BBC's Focus on Africa programme.