French troops kill deputy to veteran Islamist Belmokhtar By Adama Diarra BAMAKO (Reuters) - French special forces have killed the second-in-command of the veteran Islamist commander Mokhtar Belmokhtar in an operation in Mali's northern region of Tessalit, security sources said on Thursday. Hacene Ould Khalill, a Mauritanian known by his nickname Jouleibib, was deputy commander of Belmokhtar's "Those Who Sign in Blood" Brigade, formed when the Algerian jihadist split with al Qaeda's North African wing. The group claimed attacks on the In Amenas gas plant in Algeria in January in which 38 hostages died, and the twin suicide bombing of an Areva uranium mine and a military barracks in Niger in May. "He was killed near Tessalit," one security source said. A second source confirmed the operation had been carried out by French special forces. French Defence Ministry spokesman Colonel Gilles Jaron declined to comment. Islamist groups in northern Mali have stepped up their operations in recent months with attacks on U.N. peacekeepers and the killing of two French journalists in the northern town of Kidal on November 2. On Thursday, at least three rockets launched by Islamist militants hit Gao, the largest town in the region, residents and military sources said. No-one was reported killed. "We heard three big explosions between 5.30 and 6.00 a.m.," resident Kader Toure said. Military planes are flying over the town right now. U.N. peacekeepers have stepped up patrols in the town. The explosions follow an attack on an armored vehicle in the outskirts of Kidal that wounded three soldiers on Wednesday, according to a source involved in the French military operation. The latest violence has sown concerns over security in northern Mali before legislative elections due on Sunday as Mali, once a model democracy for the region, seeks a return to order. The vote will complete the democratic transition following a military coup in March 2012 which plunged the West African nation into chaos, allowing separatist and Islamist rebels to seize its north. France intervened militarily in January, breaking the Islamists grip over northern Mali's towns. The instability in the north has raised questions about France's plans to cut its troops in Mali to 1,000 by February but Paris has insisted it would stick to its timetable, leaving security largely to Malian forces and U.N. peacekeepers. As part of a bid to return to normalcy, the government has urged thousands of northern residents temporarily residing in the south to return home. Advocacy group Refugees International on Thursday called this "premature." "The security situation in northern Mali is still precarious and basic services are lacking," the organization's Michelle Brown said. Another big challenge faced by newly elected President Ibrahim Boubacar Keita is to impose order on the military following incidents of army violence. Mali had summoned the leader of last year's coup General Amadou Sanogo for questioning over the death of six soldiers in September. But a judicial source said he did not appear before prosecutors this week following a summons. Additional reporting by David Lewis in Dakar, Tiemoko Diallo in Bamako and Nicholas Vinocur in Paris; Writing by Daniel Flynn and Emma Farge; Ediitng by Angus Macswan