BAMAKO, Oct 7 (Reuters) - The death toll from an attack by Islamist militants on soldiers in central Mali this week has risen to 16, the army said on Thursday, after previously reporting nine deaths. Wednesday"s attack - one of the heaviest losses suffered by Malian troops in recent months - highlights worsening security in Mali despite efforts by local, European and U.N. forces to counter armed groups linked to al Qaeda and Islamic State. read more This month U.N. Secretary General Antonio Guterres sounded the alarm, saying Malian authorities overseeing a transition after a coup last year were failing to follow through on promises to improve security and prepare a return to constitutional rule. "Progress has been limited and the situation remains fragile, with a need for more determined efforts to address the challenges at hand," Guterres said in a report to the U.N. Security Council, dated Oct. 1. He noted his great concern over delays in preparations for presidential and legislative elections that interim authorities originally said would take place in February 2022. Mali"s progress back to democracy following the August 2020 overthrow of President Ibrahim Boubacar Keita is being closely monitored in a region that has experienced four coups in 13 months, two of them in Mali. Under pressure from the Economic Community of West African States, Mali"s new military leaders agreed to an 18-month transition, but in September the authorities said the elections could be delayed. read more "Delayed action will fuel political and social instability and allow extremist groups to continue to expand their control over swathes of Malian territory," Guterres warned. The 15-member Security Council is due to discuss the situation in Mali later this month.