Rwanda-backed M23 insurgents shot dead three boys who had picked up guns left by Congolese soldiers fleeing the rebel advance in eastern Congo, the U.N. said on Tuesday, adding the incident to a list of grave human rights violations reported during the conflict.

Government forces and local militias have clashed south of the provincial capital of Bukavu in eastern Democratic Republic of Congo as troops fall back in the face of an advance by Rwanda-backed M23 rebels,

Rwandan-backed M23 rebels advanced on Sunday into the centre of Bukavu in eastern Democratic Republic of Congo, meeting little resistance from government troops, witnesses and the armed group said, as Congo accused Rwanda of ignoring calls for a ceasefire.

M23 rebels march through the centre of eastern Congo’s second largest city early Sunday as some onlookers cheer their arrival.