Volker Türk said in a statement on Tuesday that his office “confirmed cases of summary execution of children by M23 after they entered the city of Bukavu last week”.

“We are also aware that children were in possession of weapons,” he said.

He provided no details or did not refer to specific events, but UN agencies have previously accused both Congolese government forces and the rebels of recruiting children.

The United Nations Human Rights Council earlier this month launched a commission that will investigate atrocities, including rapes and killings akin to “summary executions” committed by both sides since the beginning of the year.

The M23 rebels on Sunday captured Bukavu, the city of 1.3 million people, after seizing Goma, 100 kilometres to the north last month. At least 3000 were reported killed and thousands displaced in the Goma fighting.

Early on Tuesday, the bodies of more than 20 people were recovered in Bukavu, François Moreillon, from the International Committee of the Red Cross, told The Associated Press.

The M23 is the most prominent of more than 100 armed groups vying for control of eastern Congo’s trillions of dollars in mineral wealth that’s critical for much of the world’s technology.

The rebels are supported by about 4000 troops from neighbouring Rwanda, according to UN experts.

Rwanda accuses Congo of enlisting Hutu fighters responsible for the 1994 genocide of minority Tutsis and moderate Hutus.

M23 says it’s fighting to protect Tutsis and Congolese of Rwandan origin from discrimination and wants to transform Congo from a failed into a modern state — though critics say it’s a pretext for Rwanda’s involvement.

Unlike in 2012, when the M23 briefly seized Goma and withdrew after international pressure, analysts have said the rebels this time are eyeing political power.

The decades-long fighting has displaced more than 6 million people in the region, creating the world’s largest humanitarian crisis.