Kinshasa - The United Nations said it is "gravely concerned by the use of lethal force" by Congolese security forces during a public protest in which a young girl died. The defence and security forces reportedly fired warning shots to disperse demonstrators in Bukavu in the east of the country on Monday, according to MONUSCO, the UN's peacekeeping mission in the Democratic Republic of Congo.It said an eight-year-old girl on her way to school was reportedly hit by a stray bullet and subsequently died.Locals were protesting against instability and repeated cases of armed robbery."Defense and security forces have an obligation to use force only as the last resort...," said Maman Sidikou, the head of MONUSCO on Monday."Alleged violence perpetrated by protestors should never be an excuse for the use of lethal force."MONUSCO said it has received credible reports of additional casualties and the United Nations Human Rights Office is investigating. General Louis-Second Karawa, police chief of the South Kivu province, said Monday's protests had left a 14-year-old boy dead and 53 people injured, including nine police officers. The police station and local administrative office in the district of Panzi was also set on fire. Bukavu is the regional capital of South Kivu province, which lies on the border with Rwanda.The east of the country has been prey to attacks by rival armed groups for more than 20 years.