Kinshasa - Human Rights Watch called on Tuesday for determined efforts to bring warlord Ntabo Ntaberi Sheka to justice for mass rape in the Democratic Republic of Congo four years after authorities ordered his detention."An arrest warrant alone won't stop a rebel leader like Sheka from committing atrocities," said Ida Sawyer, a senior researcher for the New York-based watchdog."The army and UN peacekeepers should increase their efforts to arrest him before more civilians suffer."Since authorities issued the warrant for the leader of the Mai Mai Sheka tribal militia in 2011, his force has killed at least 70 people, many of them slaughtered with machetes, according a Human Rights Watch count.Systematic rape"In some cases, Sheka's fighters mutilated the bodies of those they killed and later paraded the body parts of their victims around town, while chanting ethnic slurs," according to the rights group.The call for Sheka's arrest came as UN troops from the 20 000-strong Monusco force in the DRC took up positions Tuesday to support a government offensive against another rebel group - the Democratic Forces for the Liberation of Rwanda (FDLR) - in the country's restive east.The international community last July gave the Rwandan rebels until January 2 to give themselves up or face military action. While hundreds have given up, most of the fighters have dug in and refused to surrender.Human Rights Watch (HRW) says Sheka's group, also active in the east, terrorised local people with systematic rape and forced conscription from communities in the unstable Walikale and Masisi territories of North Kivu province.Crimes against humanitySome of the worst attacks occurred between August 2012 and November 2013 in and around the town of Pinga, from which Mai Mai fighters abducted dozens of women and girls, many of whom are still being held hostage as sex slaves, HRW said.DR Congo authorities issued the warrant for Sheka's arrest after an attack in which the militia under his command and two other groups raped nearly 400 people in 13 villages between July 30 and 2 August 2010. They razed almost 1 000 homes and businesses and led about 100 people off into forced labour.Due to the rape accusations and other acts that could constitute crimes against humanity, Sheka is subject to UN sanctions including the freezing of his assets and a worldwide travel ban.He has almost been arrested several times, but has always avoided capture with the help, on at least one occasion, of a tip off from sources inside the Congolese army, HRW said.The government and UN representatives have also met with him on three separate occasions since 2013 to encourage him to surrender and take note of his demands."They made no attempt to arrest him during these encounters, and officials later said it would have been too dangerous to do so," said the group's statement.