Rwanda denies sending troops into neighbouring Democratic Republic of Congo, where U.N. investigators said in December thousands of Rwandan soldiers had been deployed in support of the M23 rebel group. The rebels took control of the city of Goma in the volatile, mineral-rich east on Tuesday.

M23 rebels in the Democratic Republic of Congo were reported to have taken the key eastern city of Goma in fierce fighting with government forces that sent streams of refugees fleeing area communities.

The United Nations expressed deep concern at reports of M23 rebels and Rwandan troops advancing south towards the Congolese city of Bukavu on Thursday, as the militants sought also to assert their control over east Congo's largest city Goma.