Four United Nations workers who had been taken hostage by militants at a hotel in central Mali were freed on Saturday following a deadly standoff with security forces, according to a Defense Ministry official and a U.N. spokesperson.At least 12 people were killed in the standoff, which began on Friday at the Byblos Hotel in the town Sevare, located some 600 kilometers north of capital Bamako.According to local military sources, the casualties included five Malian soldiers, three hotel employees, three of the hostage-takers and a U.N. contractor.A local diplomatic source told Anadolu Agency that the Byblos Hotel is known for frequently hosting U.N. personnel.The same source said that Ukrainian pilots working for MINUSMA (the U.N. Multidimensional Integrated Stabilization Mission in Mali) had been among those taken hostage.A Malian security source on Saturday told Anadolu Agency on condition of anonymity that the government suspected the Ansar-ud Din group of being behind the attack.The group had claimed responsibility for two previous attacks in northwestern and southern Mali on June 27 and 28 respectively, in which a total of six people were killed.One week earlier, on June 20, the government and a Tuareg insurgent group -- considered Mali's largest rebel organization -- signed a peace agreement.However, Ansar-ud Din, which is based in northern Mali, declined to attend the negotiations that led to the deal.In 2012, violence erupted in northern Mali following a failed coup attempt and a Tuareg rebellion that allowed al-Qaeda-linked militants to take over the northern half of the country.In early 2013, former colonial power France sent troops to the North African country and -- with the help of Chadian and other African forces -- flushed the militants from Mali's main northern cities.Nevertheless, recent months have seen sporadic militant attacks on U.N. peacekeepers and Malian army personnel.In July, six U.N. peacekeepers were killed in the northern city of Timbuktu.And last week, 11 Malian troops were killed by unidentified gunmen in an attack on a military camp in the Gourma-Rharous area some 160 km east of Timbuktu.