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,

Burundi is withdrawing its forces from eastern Democratic Republic of Congo where they had been fighting against M23 rebels, four sources said on Tuesday, in a further blow to Congo's army as it struggles to halt a rebel advance.

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