Kidnapped aid workers released in Niger, one killed By Abdoulaye Massalatchi NIAMEY (Reuters) - Five African aid workers kidnapped last month in central Niger have been released after being held captive in the deserts of Islamist-controlled northern Mali, officials said on Saturday. A sixth aid worker - who was also kidnapped - was shot during the abduction and later died of his wounds, the officials said. "The violent death of our colleague Aime Soulembaye is an unjustifiable tragedy," said Sani Sayadi, director of Nigerien aid group BEFEN, which helps women and children in the impoverished West African state. BEFEN and its partners are, however, relieved to learn about the release of its other colleagues. Gunmen linked to al Qaeda factions operating in the Sahel and Sahara zone have kidnapped people in Niger and taken them to neighboring Mali in the past, though they usually target Westerners for ransom payments. The threat of kidnapping has slowed investment in the country, the top supplier of uranium to France's nuclear power industry. French firm Areva delayed the planned start up of its Imouraren mine in Niger after seven of its workers were kidnapped in 2010. Unidentified gunmen kidnapped the six aid workers last month, who included four citizens of Niger and one from Chad, from the town of Dakoro in central Niger overnight on October 14. A government official said the five freed aid workers had arrived in the western Nigerien village of Yassan, near the Mali border, during the night on Friday. "According to them, they were being held in northern Mali and one of their colleagues died after sustaining a gunshot wound," the official said, asking not to be named. The Islamist takeover of the north of Mali has created a security vacuum, opening up a safe haven for extremists and organized crime groups in the Sahara desert. Representatives from Niger's uranium exploration companies met President Mamadou Issoufou late on Friday to ask for tighter security in the north. "We explained all the difficulties we are facing in our work as a result of the security conditions," said Ibrahim Alasso, speaking on behalf of the companies. Additional reporting and writing by Richard Valdmanis; Editing by Andrew Osborn